So, to start, here's what I've loved:
1). Having to use my ground mount until level 90. It was amazing to actually get to feel just how giant the world was again. Pandaria feels like a giant continent, where everything flows together instead of the disjointed jumble that was Cataclysm zones. The Cataclysm zones seemed designed (especially Deepholm) for you to be on your flying mount, so it was more like tiny quest areas in certain areas, and giant nothingness in others (especially in Twilight Highlands).
2). The return of the war. Now, people who know me know that my character is a pacifist, I'm a terrible PvP'er, and I don't really ever participate in PvP. But they handled this extremely well. While I'm not fond of what we're doing to Pandaria (I felt terrible at the end of Jade Forest, but I won't spoil it for anyone), I think they really brought that feel of total war back to the game, and I hope they can keep it going in 5.1 when the war will resume in earnest.
3). Reputation. Oh my god, yes. No more chain-running dungeons just to get rep with the factions. Now you're actually doing things for them and feeling like you're actually involved in the world. I am so in love with the Klaxxi, and can't wait to help them out in Heart of Fear at the end of the month. I also love the Tillers. I didn't think I'd find "Farmville" that appealing, but it's actually been pretty awesome. Growing my own crops is surprisingly fun. Additionally, the method of gathering Lorewalkers rep is awesome. Flying around Pandaria, exploring and finding the lore scrolls, then having Cho show me what happened in the past? More of this, please. Also, flying disc.
4). Professions. For once, in my WoW life, blacksmithing was not a pain in the *!@ to level. It was GLORIOUS. There is so much ore everywhere, and I can only think that it's Blizzard's response to botters. They will make the markets so saturated that farming the stuff just isn't worth it. And even if people are botting to farm the stuff, the price will stay way down.
5). Dungeons. Contrary to what I thought my feelings would be, I am really enjoying the new model of dungeons. Yes, the heroics are ridiculously easy, but I know that challenge modes are out there, and I can't imagine what kind of nightmares await looking at some of these heroic dungeons. I haven't touched them yet, because most people are still hitting 90, but they look exciting, and their presence (and Blizzard's promise not to nerf them, along with gear scaling down from higher tiers to come), gives me great hope. There's finally something for everyone when it comes to dungeons.
Now, what has frustrated me:
1). Motes/Spirits of Harmony. Seriously, #$%^ these things. I know that once I get to exalted with the Tillers I can grow my own Motes of Harmony, but seriously, screw this. Having to use them both to make gear, and buy gear, is just a horrid design. Additionally, having to deal with these things on my gathering alts (which are now, really, worthless, because materials are so cheap) is just frustrating. Yes, I can buy crafting materials with them, but why do I need to buy them? I am practically drowning in Ghost Iron right now, and getting it transmuted into Trillium when needed isn't hard.
2). Living Steel. Especially that Living Steel is required to make belt buckles. That's just a giant pain in the -*!. I would have been okay with it requiring Trillium (like the Cata one requiring Pyrium), but having to make it using an alchemist CD (which should have been a mining one, like Titansteel, in my opinion) is a bit of a frustration.
3). So many flavor items. This isn't really a bad thing, I guess. It's just somewhat of a frustration when I'm already short on bag space. I need more slots opened in Void Storage, or Blizzard needs to devote a tech team to figuring out how to rewrite the code to resize the backpack up to 26 slots.
But ultimately, the things I dislike are tiny in comparison to how amazingly awesome the game has been. I look forward to seeing it from the other side as I level my mage, and look forward to playing my monk. So far this expansion has blown Cataclysm out of the water, and is rapidly approaching the same level of awesomeness that BC had for me. World of Warcraft may be eight years old, but I certainly feel like they've managed to capture that awesome "newness" that made Burning Crusade so awesome in Mists of Pandaria.