Just to make sure, you do know that MH3 will be online? Free, with no friend codes. Also, similar to the downloadable quests on the PSP versions, there are "Event Quests", which are special quests that can only be found online, and are only available for a certain amount of time. You don't download them, though, you actually have to accept them on the spot like any other quest.
For example, in the original Monster Hunter, once every other month, there was a quest called "GMR" where you had to slay 2 Diablos. This quest was available for a few days. What's special is that once you complete the quest, you get a rare "GMR Ticket", and upon taking them to the armory, you could forge a rare Greatsword called "GMR Chrome Razor" which is unavailable anywhere else.