All too often when somebody is describing a game they're saying things like "boy the gameplay is really good" or "the gameplay stinks" or some variation of that. What does good gameplay even mean? I think the problem is that people are using it like an umbrella term. When somebody says a game has great gameplay it probably means it has great concept, focus & direction, structure, rules, mechanics, controls, level design, enemy design, balance, design decisions, variety, fun, and so on and so forth. Trying to wrap all of that up in a single word like gameplay is simply impossible. At the expense of maybe a couple extra minutes I believe it would be more beneficial to actually describe what makes a good game instead of going by the old standby "it has good gameplay".
The concept is the simplest thing about the game but is potentially the most important. With the concept you sum up the entirety of the game in a couple lines. Through this description a basis is established and a twinkling of identity is created. Identity is paramount to making a great game because without an identity it's only a matter of time before that particular game is made obsolete by something bigger and better. Anyway concepts are simple to create but not so simple that you'd say Ikaruga is a "2D shooter" or Call of Duty is a "first-person shooter". Ikaruga is more of a "Chaining-based 2D shooter where life and death are tied to ship & bullet color." while Call of Duty is a "Checkpoint-based first-person shooter with rechargeable health and multiplayer." Again establishing an identity through concept can be useful even though it's just a small part of what makes a game great.
From the concept we go on to the focus & direction. While the concept establishes a fine basis without focus or direction it's nothing more than a concept. Is the game going to be arcade-style and feature simplistic gameplay with steadily increasing difficulty or is it going to a free-roaming game that has a variety of ways to play and multiple solutions for every stage? Some games tend to throw out a lot of ideas and play around. This is fine as long as the focus remains the same and the direction goes unchanged. For an example of a great game that likes to play around just look at Super Mario World. That game also uses a fairly non-linear map as progress though so it's a bit more understandable. On the other hand if I was playing an arcade-shooter like Dodonpachi and stage 5 was actually easier than stage 1 because the designers tried something new that didn't really build upon anything in the other stages that would be a grave issue.
Structure is the most essential aspect to any game. I don't think I've ever played a game with poor structure and that's because without structure a game is simply incomplete. It'd be like playing football but without touchdowns, field-goals, or even safeties. Without a proper structure the game may as well not even have a beginning let alone an end. For arcade games and most older titles the structure is obvious. In those games without endings you continue to play until you run out of lives and in those games that do end you try to beat the game without running out of lives and/or continues. Even with newer genres like MMORPGs structure remains an important part. There's a constant array of quests, dungeons, and PvP battles where the object is to collect X number of objects, reach the end of the dungeon and kill the boss, or complete the objective against the enemy party. The structure in something like a fighting game is simple. The person to deplete their opponent's lifebar completely wins the round. Getting bored and quitting or finding that the game is too hard are not factors in structure as we're assuming that everyone will complete every game they play. Whether they actually enjoy themselves and want to play the game again is another story though. Needless to say if a game fails the structure test it's not a game, it's an alpha, a beta, or whatever the heck a game is called when it's still in the testing stage.
The rules of the game must be established early on. The player has their own set of rules, the enemy has theirs, and the level they compete upon has its own rules as well. Unlike football or any other sport the player is usually unable to break the rules they're given. Instead they must work with what they're given and it is up to the enemies and level to reward or punish them. Rules should also properly determine small yet important things like how much damage a particular attack from an enemy does(provided the game gives the player a health meter). If one attack does more damage than another then there should be an understandable reason as to why that is. When the game punishes the player for breaking a rule it tends to come off as a design decision. These should be avoided if possible as they go against one of the most important aspects in a videogame which is balance.
Along with the rules there must be mechanics. In this blog I talk a lot about mechanics and for good reason. Mechanics dictate how everything behaves when actions are performed as well as how everything reacts when an action is performed on them. When you're playing a beatemup what should do the enemy do when you punch them? What about when you perform a jumpkick on them? When you go to grab them is that through a button-command or by getting close to them? Should the player approach from a certain angle to maximize effectiveness? Strong mechanics can be a lot of work because there's a ton of aspects to consider. The great thing about mechanics though is that it's one of those aspects that make up a game with a good identity and therefore a great game. Mechanics is what separated Mario from other platformers of that day and what separated Sonic from Mario. Mechanics can also cover other aspects like hit-detection. Say you're playing a 2D shooter and the game expects you to fit your large ship in-between a bunch of bullets. Usually this is accomplished by making the hit-box smaller than the ship itself. Chances are if your ship can't fit in-between those bullets it's because it's either against the rules and you weren't supposed to try that in the first place or there's a problem with the mechanics.
Controls is at about the level of structure in terms of importance in a videogame. Basically if the player can't control their character with maximum effectiveness while staying within the set rules and mechanics the game is broken. Controls must be fluid as well as intuitive and depending on the genre the player should be able at least accomplish basic tasks with little effort. A combination of rules and mechanics will limit the player's controls but if they are hindered by aspects that are not considered sensible than there is a problem with the game. Basically if your player is slipping all over the place and he's not in an ice stage or the enemies didn't cause some "slippery movement" effect then something is seriously wrong. Sometimes controls might seem limited compared to similar titles but that's not really a problem. In fact a videogame is not made to be compared with another game. Above all it is a vision and either the player deals with it or they go and find a game that's more suited to their taste. In the effort to create an identity and be remembered as great it is not the game's duty to provide for everyone. I didn't mean to get sidetracked there but it's just one of those things that bugs me when people talk about certain aspects of particular games.
At this point we've reached one of the more particular aspects of a videogame. Level-design is an umbrella term in itself as it covers more than just the average stage in Super Mario Bros. Level-design can extend to fighting games(yep even the ones that are just a simple rectangle with different backgrounds), strategy games, MMORPGs, Grand Theft Auto and its many clones & variants, and you get the idea. From the level-design all of the enemies, rules, mechanics, and every other aspect of the game is put to the test. Through proper usage of level-design the game can apply new rules and enemies as well as introduce greater challenges or work things out however it would prefer.
The enemy can be anything. They're the goombas you bump into, the ninja dogs that surround and annhiliate you in Shinobi, the ships and tanks that fill your 2D shooter with bullets, the enemy base that's controlling all of the squadrons out to get you in a Strategy game, and so on. Enemy-design determines everything from their behavior to their means of attack. While they tend to be straightforward(especially in older games) in newer games they require a lot of AI routines to deal with all of the freedoms they've been given. On that same note however depending on the direction the developer wants to go in they can put in more "dumb" enemies as opposed to a few "smart" ones to either change the rules for a particular stage or in extreme cases develop the entire game around them. Great games can do one or the other but rarely can they accomplish both.
Throughout everything there is one aspect that must be maintained and that is Balance. It is afterall the nature of all things so we expect all great games to have balance. All strong enemies must have a weakness just like all of the best weapons have limited usage and the riskiest move is usually the most rewarding. It is entirely possible that this aspect of a game would require the most testing and even today it's not always guaranteed. Regardless it is just as necessary as anything else.
Design-decisions are a tricky concept to grasp. I'd like to think that most if not all of them are arbitrary and are nothing more than a quick-fix to get out of balance issues. Trouble is these are usually hard to identify and harder to prove that they're actually design-decisions. Off the top of my head I can remember one example though and that is in Final Fantasy Tactics Advance. If you've played that game you already know what I'm talking about. The judge-system is at best inexplicable. The game decides the rules for nearly every battle and punishes the player severely if they break them. There is a way around this but it's not sensible. At least with the sequel the judge system was revamped so that the only punishment for breaking the rules was the loss of a few prizes and the ability to revive player-characters. I'm not saying it's the optimal solution but rewarding the player for playing by even the strangest rules is always preferable to punishing them for breaking them. Regardless design-decisions are something to look out for.
Variety is basically what we determine to be when and how often a game changes. Most of the time variety extends as far as the level and enemy design but sometimes the rules and even the genre can be changed as well. The things that should go unaffected despite this are the mechanics and the concept. One example that readily comes to mind as a game that changes its focus yet retains nearly everything else is Offworld: Stranger's Wrath. Without giving too much away(though you're doing yourself a disservice if you haven't played the game by now) the game goes through a change that could be conceived as radical yet it still retains most of the qualities it had when the game first started. It's a credit to the developers when variety works well but at times it's more of a crutch to make up for some quality that the game is lacking in.
But is it fun? This is the wildcard of a videogame and can potentially over-ride everything else. Establishing what makes a game fun is not an easy task and relies solely on the player to determine for themselves. Furthermore the only real way to share what makes a game fun is to relate experiences the player had with it. You could watch a video of a game or listen to a friend tell you about all of the fun things he did in the game but in the end there's no real way to develop fun. It's only through the combination of all these aspects and how the player respons to them can they conclude for themselves whether something is fun.
Obviously all of this isn't going to translate well to a single word. Whether you bothered to read all of this or not I implore everyone out there to say what makes game X good aside from saying it's the gameplay. In truth gameplay has no actual meaning. It's like saying the game has good graphics or sound. Sure it was fine back when we were all reading Electronic Gaming Monthly and reviews were done in fifty words or less but with the freedom that the internet provides we can easily spend hundreds of thousands of words simply talking about the mechanics or level design of a particular game.