There's a lot of different answers here.
First I should point out, we do rely on players like Docaholic... and Vhab was once a player and we hired him (we generally look to the community when we hire). There is also Gridstream, ao universe, Demoder's tools, the various maps, etc. Even things like bots. I am told players were heavily involved for the Profession Changes.
Second of all, if you mean letting players actually make content (even social items) our tools just don't support this. Hypothetically, you could design a game from the ground up around User Generated Content (I once worked on something like this)... but to quote conventional wisdom, 90% of user-generated content isn't very good. So on that project were looking at a website, and a rating system, and you could vote to get certain things in the game... but that's a whole ball of wax.
So consider the following, and I'll let you in on a bit of a secret. In my experience when you are a "creative" at a game company -- and by this I mean, anything from designer, to design sub-lead, to lead designer (I have been all of these) -- the worst possible boss you can have is basically the equivalent of your average forum poster, and WoW is the most common example because everyone has played WoW.
I can't tell you how many companies and projects have been thoroughly ruined because the CEO or Executive Vice President or whomever is at the top of the food chain at that particular company plays some WoW and suddenly he's a design expert. While a lot of people are creative, being a game designer is a profession and it takes a combination of talent, hard-work, and the most important thing is experience. And by experience I don't mean on paper -- I mean actually making something, and realizing what it takes and having a sense of what the tradeoffs because there are always tradeoffs in design and technology, and scheduling.
I make it my mission to read the forums at any place I work, because players can often have ideas with some "seed" in them that can be grown into a great workable idea. That's what a designer does. And I work with the player-volunteers whenever feasible. Here we call them ARK, Professionals, Council of Testers, etc... they have different names at different places but it's basically the same thing. Because let's face it, players are experts in how the game is. But being an expert in how a game is doesn't imply expertise in where the game should go, or even how to get there. Sometimes players come up with some really great stuff, which is why we read the forums.
But trust me you do not want us to "rely" on the community's creative ability when it comes to anything design related. The fact is, there are a lot of moving parts to an MMO and most players are very myopic (they care about what's in front of them at the moment). If we did this, you could get a mish-mash of random features that certain people thought were "cool" but didn't fit together in any sensible way. But primarily, nothing would ever get finished. Projects would be half-started and abandoned when something else shiny was presented.