There are a fair number of games that feature content creation as a major play element. There have been some pretty much since we figured out how to draw space invaders to a computer screen. Let's talk about the way games integrate content creation.
In the "classic" style, games integrate content creation as a stand-alone editor and/or importer. For example, Pinball Construction Set in 1985 (one of my favorites as a kid) had a level editor you could play with. It's become pretty typical these days.
Many modern games feature an edit mode switch in the game proper. For example, Little Big Planet and Second Life both feature an "edit mode" where you can create and alter objects from wherever you're standing in the level.
But the method steadily gaining in popularity is when the edit mode is tightly integrated with the gameplay: as you play the game, you edit the game. An obvious example is Minecraft, which is basically impossible to play without editing the level. In such cases, you initially use the editor to survive and explore a bit, but once you get good enough, you begin to use the editor to do increasingly self-driven things, such as building a giant garden or the USS Enterprise.
Another example of integrated content creation is when the act of playing the game ranks content and makes other players more or less likely to see it. Later, I foresee a fair number of games where you'll run into random mutations of content and rank it simply by playing, allowing a fairly large number of players to participate in an evolving ecosystem of content. So you don't absolutely have to be creating content yourself: ranking and sorting content is a valuable form of content creation in and of itself. Well, assuming you have the requisite foundation of content to rank and sort.
In my mind, this final form of content creation is the future. Integrated content creation. It has the highest "pull" - you'll get more content per player (on average) and you'll get better sorting and distribution if you use integrated creation and ranking. Much more efficient than mode-swap content creation, and almost infinitely more efficient than level-editor content creation. (To be fair, I think most games will use all three to some extent: they have different specialties.)
Since content creation / player-sorted content is such a powerful tool, I expect to see more of it in future games. Therefore, how to do it best is also an important topic.
I'm not as expert as I would like to be on this matter, but here are three areas I feel should be on everyone's list:
Base content creation: how does your player base create content as they play? The more restrictive the framework, the lower the barrier to entry... but the lower the maximum quality of the content will be. Minecraft uses a rigidly square grid and almost no moving parts, allowing players a fairly easy time of it, but also crippling some high-end content. For example, no NPCs, no elevators, no swinging pendulum traps...
Distribution and sorting: how do your players share content? The more helpful your distribution system (IE marketplace), the happier your moderately skilled player base will be... but it will also be very crowded and full of crap. A sorting mechanism driven by player preferences can be very handy, but can also brutally suppress starting players who are still getting used to the editing system.
Content cooperation: how can your content be used in relationship to other content? For example, create a shirt, and you can put that shirt on any humanoid frame. That's cooperative content. The more ways content can interact with other content and/or rules, the deeper the system will be. If you create an axle with wheels, can you only use it with cars? Or can you staple a hundred of them to the bottom of a house? If you create a shirt, can it be torn up, made wet, transformed by the environment, mutated, tailored, or painted? Obviously, the more kinds of interactions content can have, the more potential to draw players in.
The primary drawbacks are A) the amount of time it takes to code those interactions and B) accidentally forcing players to wade through details when they just want a quick result. IE, if they're making a skyscraper and you force them to choose every door, every carpet... that's a big barrier.
Anyway, that's content creation as I see it. You?