It's interesting to me how this same pattern is repeated for nearly every WoW expansion I've experienced (which is currently all of them). During the expansion, it seems the masses decry and criticize the current content for any of a number of reasons. In TBC I heard complaints about barriers to entry for raiding, the 10-man to 25-man split when it came to going from 10-man Karazhan to 25-man Magtheridon/Gruul/etc., and even the oppressive difficulty of heroic dungeons like Shadow Labyrinth in starting iLvl gear. Along comes WotLK and almost overnight TBC is given the rose-colored treatment... with criticisms ranging from the dumbing-down of the game due to easier heroics, Death Knights ruining class balance forever, lackluster raids, and a general sense of anti-climax with ICC and RS.
Now, in Cataclysm, we see the same cycle... with oft-recurring sentiments like "Tell them only that the Lich King is dead, and that the World of Warcraft died with him." TBC and WotLK are seen as almost on equal footing and nearly every element of Cata is roundly detested (if you listen the vocal minority, in any case). I'd be willing to make a generous bet that when MoP rolls around we'll suddenly see people extolling the virtues of Cata with gems like "man, raids are so easy now, at least in Cata there was a challenge" and "this expansion is for babies; at least Cata was brutal/gritty/real/whatever."
I've no idea what causes this kind of cycle of behavior... I understand the whole rose-tinted glasses hindsight thing, but I just don't get the "why" part of it. To me, Cata has some glaring flaws, and it has some strong merits... it was, all in all, a good expansion. It's my hope that the developers will take the lessons learned and apply them MoP in a positive fashion, all the while adding all new things with the potential for mistakes along the way.