Gameplay is simply the actions a player is allowed to perform in a game. If those actions are enjoyable and the controls are intuitive, you have the most important ingredients of a great game. -Duane Alan Hahn
At a fundamental level, that is what gameplay is. What you do in a game, what you can do, what you're allowed to do. It's all about how you interact in the world you've been dropped into.
Strangely, gameplay often takes the backseat in todays games which I find absurd. A backseat to grahpics and visuals.
We used to spend so much of our time on game play and today's games seem to put too much emphasis on graphics and sound. It's the game play that makes a game fun, sometimes they forget that. - Larry Kaplan
A good example of this is God Of War 3. Sony spent several years, crunching at the end to bring us this third installment of Kratos' blood frenzy. I was an avid fan of the God Of War series from the get go, loved it – no other game was like it. At the time.
Then they brought out a second one and yeah, it was alright, I guess. Pretty much the first game just with a couple new weapons (which were made obselete by the default blades Kratos wields from the get go). Still, a nice game with nice new areas etc. Gameplay is exactly the same as the first one, only change is visuals.
Several years later the long anticipated God Of War 3 for PS3 is released with high expectations. Again, the visuals were upgraded to a spectacular level not just in terms of graphics but on scale; there really were some incredible scenes in that game. Gameplay? Yup, exactly the same. You are Kratos, you press square and triangle in an orderly fashion until things die in glorious gysers of blood. New weapons, again made redundant by the default blades he has in the start. Sure, they have their strengths but why bother? It just means spreading out precious experience points where you could simply jack-up one weapon.
After the second instalment I was a little on the fence with the series as to whether I was a fan or merely a guy that would say 'Hi' if I saw them at the bus stop.
After the third game I could really give a shit. Wasn't a BAD game, just uninspiring and didn't pack that punch the first did.
All because visuals took priority over gameplay. Another series of games that's met much critical acclaim with a similar ailment is Uncharted by Naughty Dog. Same story as God Of War. Gorgeous visuals but same old gameplay: Hide behind waist high cover and unload your gun into baddies who are also hiding behind said waist high cover. With all the other games out that are doing the whole cover-to-cover shooting, this leaves much to be desired. Again, the same can be said with God Of War and the 'simon-says' style button mashing.
I'll end this part with two quotes that sum up my feelings towards the game industry:
It's like these developers are trying to invent chess and have created a superb, glossy-looking board and a whole new set of exciting pieces and then sit back and say, "Look! Look at his new board game we've made! Look at these shiny pieces and this state-of-the art board! What a great game this is!" - Neil West
Many people in the business today seem to be more interested in making movies than in making games. - Tim Skelly
To expand on what gameplay is: I'd say it's most basic function is to appeal to you, the player, on a sensory level to generate an emotional response.
The most important part of game design, including it's fluid and unhindered generation, is the design document. A documentation created by the designer on what will go into the game largely based around what things will look like and how things will work. For example, a character idea should be at a level of detail as to let the artist read it and fully understand what they have to create without the need to track down the designer and interrogate them upon every query.
The purpose of this document, as stated above, is to make the ebb and flow of game creation as smooth as possible with as little setbacks as possible. Though this doesn't always (if ever) seem to be the case as design documents are often flawed; either lacking information or overburdened with it. Deadlines are also stated here.
The structure of the document is also highly important; it's contents should be labelled and indexed allowing anyone to merely pick it up and find what information they require without having to read through it as if it were a novel.
The inception and creation of the document is down to the designer however it is shortly thereafter put to the criticism of the lead artists and programmers to gauge it's plausibility. i.e. can this team of artists accomplish the amount of work marked out?
For me, what I find important in games depends solely on what the game is trying to accomplish. For example, if it's multiplayer only with the gameplay based around you and your team winning a battle, like League Of Legends, then I don't care for narrative or story, as the games sole purpose is multiplayer and gameplay.
On the other hand, in games like Bioware's Mass Effect or Knights of the Old Republic, story takes priority over gameplay. However, the gameplay in both of those games, to me, is pretty damn aweful. I cannot stand pausing during combat to select my abilities or to have a sort of, turn-based combat. That really ruined those games for me, story was pretty good however gameplay was horrid.
Then we look at something like Oblivion, by Bethesda. The story was alright, nothing spectacular and awe inspiring however the gameplay was simple; click to swing your weapon/cast spell. Simple, doesnt get in the way, anyone can pick it up. Of course that's a little stale but it doesnt get in the way of your enjoyment of the game, you just click. Nice.
I didn't play Oblivion to be pressing 20 different keys in a precise order. I played it to enjoy the scenery and embark on some woefully trivial rpg quests. Yet if the gameplay required me to press 20 different keys then I'd likely not enjoy it, as doing those trivial quests and a bit of sight-seeing would require lots more concentration and therefore dwindle my enjoyment of this simple game. I play beat'em ups if I want to mash the controller; you expect that from them.
Therefore, what's important to me when I play a game is relative to the game I'm playing.