I quit for the fifth and (most likely) final time several months ago. Cataclysm was neat and had a lot of new ideas, but I hit 80 VERY swiftly. The challenges went from easy (or nonexistent) to insane with heroics. Not so much because the heroics themselves were hard, mind you, but because they were designed where people actually had to pay attention and use all the abilities they were given and it seemed like NOBODY I grouped with for the relatively few I did was doing that.
Once I hit that wall, and I realise that I had actually done everything else I could do aside commit my life to a guild for raiding, I knew my time was finally at an end.