A reprint of my response to York's post here: http://tatteredpage.net/archives/65
I don't think the best avenue of slowing content consumption is to encourage player-made content...though the social inventions of players (strategy sessions, group events, player ceremonies, etc) are very useful to that end.
Instead, the best route is increasing replayability through variation within static content. It's an old challenge, and one developers have tried and failed to overcome numerous times, to make content less linear, but it is achievable. SWG allowed a great deal of lateral exploration, and that game didn't explore half the methods I would use.
Here's an old example of mine: Imagine that players are exploring an Egyptian-like tomb full of statues of various sizes and representations. Each of the statues are inanimate until triggered to come alive by player presence or actions. Each has a different probability of being triggered, ranging from very unlikely to very likely. Those probabilities are not static...they are adjusted weekly, monthly or however often the developers desire (and those adjustments could be randomly determined by software, so the devs would not have to spend time on that). This means that the same player can explore this same tomb many times and continue to enjoy fresh experiences in doing so (assuming that other elements of the tomb experience vary as well). It also creates uniquely individual player experiences for memory (important to player retention) and social discourse.
Variation can make old content into fresh content. In Mario Kart, I enjoyed the same racetrack countless times because there was enough variability to make it fresh. In Counterstrike and Gears of War, players enjoy the same maps over and over because of the variation brought by their fellow players.
In MMOGs and RPGs, replayability in small experiences, not just the cumulative adventure (i.e., playing as an undead warlock instead of an orc warrior), can greatly reduce the average rate of content consumption.
That is not to say that it will prevent the most fanatical achievement-oriented gamers from continuing down a linear path. But personally, I don't consider that small population to be a great problem as long as you've got enough loyal, equally-vocal fans to counteract the publicity impact of ranters. The majority of achievement-oriented gamers, who are not so fanatical, would be able to enjoy the replayability.
Imagine: The first time your group traverses the main hall of that tomb, the avian statue and the wolf statue come alive for two separate battles. The second time through, four statues come alive, two of those at the same time for a bigger battle. The third time through, only one statue comes alive, but you also trigger a boobytrap that makes the single statue more difficult to fight (combinations of triggers could be limited in advance by developers to control the range of difficulty...though some of that control could be offered to individual players). The fourth time through, 6 statues come alive, three of them together and one of them a particularly formidable foe...but a trapdoor is discovered, allowing the group to escape.
That's the sort of gameworld I want to play in. =)