As for the thread topic, MoP is for me the best expansion. What I did not like however:
- Valor gear double gating: There is already a gating (the cost of the piece). There should not be additional one imo. I hated the decision to gate the 5.2 valor gear to raid reputation. It forced me to run LFR just for the sake of getting rep. That's a bad design. Even if I love to daily quests, I think it should not have been gated to faction reputation in 5.0. neither. If they really fear we would gear up to quickly, just increase the valor cost of those pieces.
-A minor nitpick: Horde and Alliance main hub should not have been so close of each other. They are at war and we hear they are living "peacefully" at close range in the Vale? They should have spreaded out the 2 cities in different zones.
-Alt unfriendly. The expansion has been a bit alt unfriendly. They added nice elements to promote alt play like faction commendations to increase reputation or account-wide mounts and pets or the 50% valor buff. The next step should be to put secondary professions (or some of them like fishing and first-aid) account-wide as well.