I quit because the game dragged on and barely evolved from what it was when I first started playing over a year before.
I got to Kel'thuzad and never bothered trying him because it was more of the same boring crap. (Except this time you needed 10 potions per try, yay)
I hardcore raided Molten Core, BWL, AQ40 from start to finish and nearly Naxx from start to finish. (Downed 4HM then got bored)
Nothing to do with effort, it's just generally a very dull game after a while, killing the boss first can sometimes be fun, then after that it's just tedious.
The game was 10x better when it was just MC/BWL/AQ40/20 and Zul'Gurub, even though it was the same bosses I actually enjoyed them instances, especially late night runs where wiping wasn't taken so serious as it was in Naxx. (Granted I did enjoy Naxx but needing so many potions per attempt on certain bosses REALLY killed it and got boring)
I gave TBC a go with some friends and it was the same, leveling was enjoyable but when the server had evolved a fair bit that guilds were raiding properly and I had tried numerous heroics and other instances I just got bored again because it was more of the same.