Because the last patch (dragon soul) is one of the worst content ever in this game (if not THE worst) and we got stuck with it for a whole year, and that's what people will forever hold from the expansion.
Recently I leveled another alt and for the first time going back to that content since MoP came out. Vashj'ir, Deepholm and Uldum are, in my opinion, some of the best zones ever. Hyjal is ok and Twilight Highlands is a bit boring (and the last, so, like before, not good to keep good memories alive), but the rest of the Azeroth overhaul is superb.
That, launch raids (Bastion and Blackwing), Firelands (the whole patch, not only the raid) and the whole streamline of gameplay was some of the best things Blizz made for this game. Also, Cataclysm is my favorite WoW soundtrack so far.
Besides, the whole apocalyptic-fiery-violent and chaotic feeling to it just seemed right at the time (after the alien TBC and the Dark WotLK).
This is just a rambling on a underrated content, spoiled by the major failure that was 4.4.
Hope WoD will come soon enough so we dont get burned out by SoO aswell (i dont think that will hapen, though... we were stuck with ICC for a whole year aswell but it was awesome, instead).