I'm almost 88 on my priest and loving the new content. The new PvE is pretty interactive and a lot less reminiscent of the old kill 10 mobs, gather 10 branches and report back to me. There's still a bunch of filler quests, but you don't really notice. I'm also invested in the story line of an MMO for the first time in a long time. What can I say, the story is good.
As for the PvP, this expansion is AMAZING. If I could describe the world PvP in the mists it would be Mordred where half of the server is in your guild. Since it's such a small space you run into enemy faction players all the time. People assjam you when you're doing encounters or turning in quests, it's awesome. Constant need to be on your toes. Plus there are plenty of WoW carebears around to gank. Weirdly, I haven't seen any 90s ganking so far. I expected to see a lot of that in the 85-87 zones, but so far I haven't fought a single 90. Anyway, I haven't done any BGs or Arenas or anything like that, but just the world PvP is keeping me entertained. If that's your thing, I highly recommend it.
I was a little apprehensive about the new talents, but I gotta say they nailed it. You unlock a new tier of three abilities to choose from every 15 levels starting at level 15. You can choose 1 ability per tier, and all three are usually powerful options. Most of the old talents have been incorporated into baseline/specialization class abilities and glyphs. Some of the new glyphs are pretty cool. For example, I have a glyph that lets me cast certain healing spells while in shadow form.
The only thing I'm not digging is the dungeons. I've done 2 so far and I'm not impressed at all. I remember either wrath or cata when it first came out (before they nerfed the dungeon difficulty) and no one knew how to do any of the encounters, me and my friends would wipe all the time until we figured out what each boss did and how to survive it. Pandaria has none of that. It's like they implemented deadly boss mods into the game itself. Sometimes it literally tells you what's about the happen in a fight. Not a fan of that in the least.