Things went pretty badly last time around -- guilds stopped raiding (what was the point, when the next expansion would replace all of our gear with greens?), players abandoned the game for a while, and there was a general depression in Azeroth. While people were excited for the expansion, the live realms seemed like yesterday's news.
Hopefully things will be better this time around -- back then, we didn't have dailies to do, there weren't any arena matches or tournaments, and the 40 man high-end raid instances were pretty much the only game in town (nowadays we have 10mans, 25mans, or Heroics, and lots of rewards from each). And depending on when Blizzard gets the expansion out, the wait might not be as bad (although if they wait until January again, the time frame should be about the same as last time). But get ready -- the calm before the storm is coming, and we won't see a new game again until we step foot on the icy shores of Northrend.