Judging almost completely by the forums, and when I see people coming and going... It looks like once you pass SM, you're either going to make it or you're going to fizzle out.

For some reason I don't see many people kicking it out after 40. Could be that instances start getting more difficult (though the real challenge starts when you try multiboxing outlands instances), could be that questing gets more spread out and there's less to pick and choose from, could be that the push from 1-40 wasn't enjoyable and there's a long road ahead of the same sort of thing. Who knows.

I nearly kicked the bucket at level 40 -- but that was because 2.3 was still in it's "nerf ele shammies spelldamage coefficient" stage and I was demoralized times 5.