For the most part I've been pretty pleasantly surprised by this expansion... but there is one thing I've found to be absolutely atrocious, and that's the way characters level up and acquire new abilities in MoP.
Initially I was going to title this post "why I think the MoP talents are terrible", but to be honest the talents themselves aren't the issue. Only getting a talent point every fifteen levels is supremely boring, mind you, and even when you get one there is no guarantee that the associated talent tier won't be boring or pointless as well. "Yes, please! Let me choose one of three crowd control abilities which I'll never use (unless I PvP) since this game made crowd control redundant two expansions ago!" But that's not the issue. I can live with boring. (And some of them are useful/interesting.)
No, my problem lies with how, in association with the new talent system, they changed the way you acquire new abilities in general. I can only imagine how obtuse the whole system must be to a genuinely new player, because even as someone who played the game for over five years and just returned after a leave of absence I found it quite confusing and irritating.
For reference, let's look at the way things used to work. There were only two things to consider while levelling up: your class abilities and your talents/specialisation. To learn new class abilities, you would visit a trainer in a city. You would see a list of what you could train since you levelled up and had time to familiarise yourself with each new ability, decide where to place it on your bars etc.
Talent points could be spent anywhere at your leisure, but there was a handy tree so you could always see how your spec would progress over time. So if you put your points into the restoration tree for example, you could easily see how each talent improved your healing in some way, with the lower tiers generally being less important and then leading synergetically into the higher tiers. ("This makes my cast-time heals faster. This gives me a new healing spell. Then this next tier adds an additional effect to the healing spell." etc.)
In Mists of Pandaria, trainers have become redundant. You automatically gain new abilities in the field, complete with a little announcement about what you just learned, and if there is room on your main action bar, a new button will appear there. So far, so good. If there isn't any room, well... tough luck. Have fun looking through the entirety of your alphabetically sorted spellbook to see whether you can find something new. While the spells you haven't learned yet are sorted by level so that it's easy to see what's coming up next, there doesn't seem to be a way to easily see what you just learned after the fact if you missed the little announcement popping up.
And oh, are those announcements easy to miss. After all it's not unusual that you'll level up mid-combat, or even in the middle of a dungeon. So every time that happens, you have to sift through your whole spellbook again to find the one ability that's new. Except sometimes there isn't even anything, since you often don't get anything while levelling up in MoP, so after a while you stop bothering every time.
I remember healing a Cataclysm dungeon while levelling up and thinking: "Man, there is a lot of AoE damage going around here, I wish I had an AoE heal to deal with it. When do I get Wild Growth anyway?" As it turns out, I already had it, had had it for quite a few levels in fact. But as per the above, I had completely missed it. The only reason I found it at all was that I knew of its existence in advance and was expecting to get it. It was also only the other week, after about three weeks of being max-level, that I looked into my spellbook to find an ability called Genesis. When did I get that? What is it? Who knows.
Which brings me to the issue of specialisations in general. Decoupled from the talent tree interface, they've been left in this awkward space where they are different from generic class abilities, but don't get any space to explain themselves beyond the generic flavour text when you first pick your spec. So the tab on my spellbook that says "feral" contains both general druid and feral only abilities mixed together. While the ones limited to my current spec do have little text underneath that says "feral" for example, it's still a lot harder to keep track of what's what when it's all jumbled into one big list. You don't get any sense of progression, of how it all goes together. And the first time you respec, you'll probably be surprised by all the abilities that vanish, because you kinda thought they were part of your class kit, not your spec's.
I feel exhausted and confused again just re-reading this. While I had fun with the levelling content, the process of keeping up with my character's ability growth has been a royal pain in the butt. I really struggled with the fact that I had no agency whatsoever in training or choosing her abilities as she levelled up; they were just dumped on my head at random intervals with no explanation, whether it was welcome at the time or not. Even now, two months after returning to WoW, I feel vaguely uncomfortable and estranged from the character - and that's coming from someone who levelled multiple druids before!
Oh, and I haven't even mentioned the glyph system. Do I even need to mention that by the time you get your first glyph slot, you won't even have ninety percent of the abilities that the various glyphs pertain to? I seem to recall leaving mine empty for quite a while simply because I wasn't given any even remotely useful options at the time.
It's just a massive headache.