WotLK was toughest time to PvE box, cata was so easy you could do it half asleep. I'm in the MoP beta but for me its still too early to tell if I will enjoy boxing it. It will probably be somewhere in between WotLK/Cata.

Blizzard seems to recognize problems, then in the next expansion they fix those problems but go too far in the other direction causing new problems.