As some people have pointed out, one of the major factors in using user-generated content is the difference between casual users who might only play a few hours per week and the hardcore users who might play as much as they work.
If just tossed into the mix, the hardcore players will generate literally thousands of times more content than the casual players can get through, and this can easily result in the casual players getting "lost" because every time they log in, the world has grown and changed significantly. On the other hand, limiting the change speed to their level bores the hardcore players.
This problem is also the basis for "mudflation", where high-end items and gameplay become middle-end items and gameplay, so each new release has to include more powerful content, increased level caps, and so forth.
I call this overall situation "content shearing". One part of the game is pushed very rapidly, the other is pushed slowly. If not handled carefully, this can result in "fractures" in the content, where casual players and hardcore players literally have no connection to each other. The content and gameplay they use are completely separated, even if the hardcore player is playing through the same levels as the casual player. In some situations, this shearing can actually tear the slow content apart, rendering the game almost completely unplayable to the casual player. As an example, see Eve Online. It's possible for the fast content to get torn apart instead, which may be the case in some relatively short-lived MMORPGs.
Right now the industry standard way to cope with content shearing is to box the content into distinct packages, essentially pre-fracturing it. This is done by gating content and segregating players. (Player segregation is a subtle thing, but primarily revolves around the idea of "guilds" or other tight groups of people who have few connections to other groups of people.)
However, gating the content to prevent shearing will only work for so long, and has some serious drawbacks, such as a huge limitation on the amount of player-generated content that can be used. So it's worth looking into some other methods that will allow hardcore and casual players to play the same game without tearing it apart.
One method is to shackle the fast players to prevent them from grinding through too fast. Some games do this by having a maximum cap on your capabilities, and hardcore players have to degrade some of their existing capabilities to learn a new ability. Other games do this by limiting turns per day, or by reducing the amount of XP gained after a few hours of play. However, these methods all share the same problem: they are telling someone to play your game less. They are actively recommending that hardcore players stop being hardcore players.
That's just silly. "Play our game, but not very much!" Yeah, great message.
There are other methods to handle this shearing. One is to use gated content, but make the gating part of the gameplay instead of something carefully controlled by the developers. In this way, you can get players to gate their own content. An example of this is in giving every character their own little chunk of town, and they can strongly affect their own bit of town. However, unless two players manually connect their regions with a street, the two areas do not affect each other very much. A casual player will find that his world hasn't changed much, or if it has changed, it's because of someone he explicitly gave permission to.
This kind of gating is more character-centric rather than level-centric or area-centric, but other variants are certainly possible, far more than I've thought of.
Another method is to pull the slow players up to the fast players instead of pulling the fast players down to the slow players. This is possible to do if you assume the characters don't stop living their lives when you log off. Casual players who log in after spending the workweek actually workweeking find a quick summary of the things their character did to keep up with the world, and perhaps what current event he's focusing on today.
This "catch-up" method requires a more carefully segmented play experience, however. In modern MMORPGs, if you tell someone you'll hunt 105,519,477,149,000 wolves for them and take a week off, when you log back in he'll still be waiting for you to grind through the quest. That's not feasible with this "catch-up" system, because in a week the situation will have changed, and being anchored in wolf-killing will make you boringly obsolete at best. Instead, each login should be thought of as its own unique episode(s) which, when you log off, finish out. If you do agree to hunt wolves and then log out, when you log back in you shouldn't be hunting wolves any more. Either the quest is off or your character did all the hunting while you were gone.
The "catch-up" method is especially handy for people who come back after long delays, as they can be reintroduced to their character and their world seamlessly, without the need for going through the tutorial again or jumping blindly into a role they've forgotten all about.
The last method I can think of is to think of content as a spinning disk. Content near the center moves more slowly and in tighter circles, while content near the rim moves at breakneck speed. Obviously, hardcore players play out at the rim, while casual players will be near the center.
The key here is that you can draw a line from the casual player to the hardcore player, and even though the hardcore player is going around faster, the line never "breaks" or "shears". The idea is to make content that stretches, such that one piece of the content can benefit from the load of thousands of slow-moving players while the other side benefits from being pushed along by a few hotshots.
This is even more suitable when you take into account the idea of players specializing in different jobs, because a hardcore player is only hardcore in one or two play styles. He's casual in all others. For example, if he's a hero, he'll be hardcore at exploring new areas, gathering teams for raids, and so on. But he's casual in terms of potions, real estate, politics, and so forth. So "flexible" chains should serve his needs in every respect without ever "breaking" and confusing him.
An example of this kind of system is in SecondLife, where a few hardcore players build the majority of the content, and then all the players casually ingest the content. In this case, content is created at the fast outer edge and "falls" downward into the casual edge. SecondLife is notoriously bad at the middle of that chain, though, as people who are kind-of good at creating content or who are good but newbies tend to have no traction on either side of the system, which is certainly not optimal.
It's also possible to have content that "rises" to the outside: casual players creating content that the heavyweights have to deal with. I can't think of a major MMORPG where this is the case, but I can think of half a dozen ways of doing it right off the top of my head, so it's not impossible.
There may also be ways of making these chains where content can't be thought of in terms of rising or falling, although I can't htink of any off the top of my head.
Some care must be taken that the falling or rising content doesn't move the chain much, because if the chain starts sliding around, we once again have problems with shearing and fracturing...
What do you think?