I hate how, for lack of a better word, unepic the game feels. In the past it was an accomplishment to be max level, kill raid bosses, get high arena rating, get your epic ground mount or flying mount, etc etc.
During Classic, TBC and WotLK leveling took time and you got rewarded on your way up. The old talent trees made getting a level feel important. Even if you put 1 skill point into a +1% hit talent your character got stronger. Now you get a talent point even 15 levels. Some of those talents are complete garbage and having to wait 15 levels to pick the only good talent is frustrating. If you wanted to level up in the past you had to quest or grind mobs. Now if you quest you blitz through the levels so fast that you'll out level a zone before you're done with all it's quests. Blizzard remastered the 1-60 questing and now you don't get to complete the zones unless you like not getting rewarded for your work.
With LFR and LFG being brought into the game killing a boss or finishing a raid doesn't feel as *cool* as it used to. When you raided from Classic through late Cata killing a raid boss was something unique to those who put in the effort to organize a raid and had the skill the kill the boss. Now you can just que into LFR, go afk, and experience killing a raid boss. Not to mention how similar the gear looks between difficulty tiers. LFR, Normal and Heroic gear look so similar its sickening. Just the thought of how Flex raid gear is going to look nearly brings me to tears.
Pvp in MoP has been plagued with insanely, over the top, strong specs in every patch. In the past there were always strong specs but they weren't so strong that you could just auto win games if you had that spec on your team. I'm looking at BM, Feral, Ele, Enhance, Shadow, Subtlety, Ret and Arms early in the expac. Some of these specs have been stronger than others but at one point in time they would just dominate pvp. Blizzard has made a bad habit of over buffing a class, leaving it OP for a while, then gutting it. Arms got this treatment, Feral is getting it's utility gutted in 5.4, Shadow had it's defensives nerfed into the ground and Hunter's stampede might as well be renamed to Petting Zoo if blizzard keeps up with their nerfing. There are also way too many super strong abilities that make pvp so hard to balance. This applies to both damage and defensive cds. Just to list a few, Void Shift, Wild Mushroom, HotW, Spirit Link Totem, Symbiosis, Chastise there is too many to list. All these super strong defensive abilities need to be in the game because of all the massive dps cds. A class pops all their offensive cds at once and you or your teammate are forced to use a big defensive cd or you just die. The things that make the game fun, seeing big numbers (healing or dmg), also make balancing nigh on impossible.
I could go on but I think that is enough complaining.
Note: I still love this game and play every single day I just think some aspects of the game were better in the past. I didn't mention any of the things that have changed for the better because its easier to pick the stuff I don't like. But many parts of the game have massively improved.