The inclusion of quests in D&D 4e was considered by some to be a nod to MMO design (one of many) but the reality has been far different. Quests tend to exist to provide alternate or extra rewards, but I have never seen them used the way they are in MMOs. That is to say, in the average MMO, a player has many quests going at once, perhaps dozens at a time, and they approach them in the order they see fit.
Historically, I haven't thought this to much of a bad thing because, frankly, most MMO quests were pretty lame. Pointless errands and requests to kill a dozen members of the monster species of the moment were the order of the day. But my thinking has been slowly changing, in large part as a result of how good a job Blizzard is doing with the role of quests in the recent Cataclysm expansion to World of Warcraft.
There is something really appealling to me in the bite-sized nature of MMO quests. Part of it is the player-directed element - you pursue the ones that interest you - but another part hinges on the size of it. You can knock out a bunch of these in a single "session", a prospect that's almost inconceivable in tabletop.
I end up wondering how much of that is habit and how much is necessity. Certain things make for concrete limits. Many MMO quests would fail to pass the smell test at the table. A "Kill 10" quest would be basically dull and stupid in that context. Similarly, RPG combats tend to be harder and more involved, so quests that depend on too many encounters are going to slog a bit.
I think back sometimes to the adventure board. Some folks may remember this as a staple of campaigns, where there was a board in town where the wanted posters went up. The idea was that there were several threads that the players might pursue if they were so inclined, but the assumption was that each such thread was a full bore adventure, not a side task. But why not?
As I think about it, I think the biggest barrier is the idea that small tasks and small rewards are less "heroic" than big payouts, but that doesn't withstand much scrutiny. Lots of studies out there illustrate that we respond really, really well to small, regular rewards that are directly related to our activity rather than deferred rewards (which become emotionally disconnected) even if the deferred rewards are larger. Lots of RPGs support this, this with more immediate payouts (in XP, action points, fate points or the like) so is it too much o a stretch ot have the setting support it as well?
Maybe. I'm still thinking. But that's a big step up from "no chance in hell."