You get certain content, like the new skills, and you get to see the Cataclysm, but you can't get the new races, participate in any of the new dungeons or do anything that the 80+ people can do. All in all, there's not much that's gonna happen by not having it. Heck, I only have Burning Crusade and I haven't missed out on anything.
Granted, that's all just speculation on my part, I dunno if that's what will actually happen.
