World of Warcraft: As the pilot to the game, there was a lot of collecting quests, 40 man raids (this could be good and bad, depending on your take), no flight, leveling was a fucking nightmare, and class design was a little ridiculous, in some cases. What was done right was that there were attunements needed to enter high end content, and something for everyone to do, no matter what level they were. There was a lot of adventure in the game, to counter the grind that it was.
The Burning Crusade: Addition of pvp stats like resilience. And then putting that on many pieces of gear. Lame. The storyline isn't bad, but doesn't make enough of a point of letting us all know Illidan really is a problem. I was a huge fan of 25 man raids over 40 man raids. Something about the ease of which a 25 man raid could be coordinated over a 40 man raid.... TBC did a lot of good things, but having legendaries drop off end raid bosses instead of being built was kind of lame. Should have called them something besides legendaries. But what did they do right? Shamans and pallies on both sides, the introduction of flying and jewelcrafting was also good. The content was difficult, and as such, the sunwell patch should have lasted a little longer. Many people never got to see it due to the fact that it was there and gone, just like that. Most of the armor was well designed.
Wrath of the Lich King: The crippling of affliction warlocks at ulduar. The fact that ulduar was so short, and ToC sucked ass. The raids should have maintained a fixed difficulty, and not differentiate between 10 man and 25 man, and then offer different levels of loot. This will come into play later for why we have a stat squish coming up in WoD. They went overboard with Arthas being everywhere all the time. But what was done right? Lots of things. Profession bonuses, the addition of glyphs, consolidation of skills, 51 point talent trees, progressionary content to get you at the end game faster (maybe a little too fast), introduction to dungeon finder, Wintergrasp (the concept, not the actual mechanics of the BG), and heroic mode (double edged sword, here).
Cataclysm: No Abyssal Maw. The dungeons at the start of Cata was too difficult compared to what they were in the last expansion. Healing stats were scaled horrifically. DS was ridiculously conceived with an easy final boss encounter, and Fuck Thrall. He has no business being in the same atmosphere as a dragon aspect. A new hero should have stepped forward there. Removal of 51 point talent trees was lame as hell. What did they do right? Expanded upon glyphs, fixed healing so it was good, made 10 and 25 man drop the same level of loot.
Mists. Almost everything.