Personally I had forecasted World of Warcraft to "borrow" the idea of public quests in the next expansion. But given the success of Rift, apparently Blizzard decided that this couldn't wait. So today they announced that public quests will be introduced already in patch 4.3.
Details are still a bit sketchy, but apparently the public quests will be tied to Deathwing attacks. So instead of just burning everything and giving players an achievement, Deathwing will open up invasion portals with the usual cast of Twilight's Hammer cultists and dragonkin coming through. Players in the area will receive a warning of the invasion, and have to work together to close those portals, before the invasion takes over the various quest hubs in the area. In the area around the portals there will be some open group system, and beating back the invasion will reward players with judgement and valor points to buy gear with.
Using Deathwing for the public quest system suggests to me that this is an experiment, because obviously Blizzard won't be able to use the same baddy in the next expansion. Although of course the principle is probably useable with most sorts of expansion final boss. I doubt they'll go back to Northrend and do Scourge invasions controlled by the Lich King though, as nobody is playing in those zones any more. Makes you wonder how public quest invasions will play out in the long term with players abandoning old zones.