This post will be coming from the mind of someone who spends the lion's share of their in-game time doing solo stuff, so bear that in mind.
I loved Cataclysm's high-end zones. Yes, they were a wee bit too linear at times, but it's not easy to get a new mindset perfect the first try. I did really like the immersion of the questing while you were questing; having everything link up, so to speak, made quest types that are otherwise less than stirring and imaginative ("kill X baddies for me, 'cuz I can't move my higher level elite ass fifty yards to wipe the area clean with a wave of my hand, 'cuz, uh... I have a bagel in the toaster and I don't want it to burn while I'm out") feel involved.
The cutscenes could get a little excessive at times, and admittedly I felt more than a little like I was in a Gamera movie during latter portions of Hyjal, but again, early attempts at new techniques (in my opinion, the whole Wrathgate / Battle for Undercity thing in Wrath doesn't count as the first try, because it was way clumsy and time-consuming by comparison).
Phasing is probably the elephant in the room, design-wise. It's a tricky line to walk between keepnig the world easily open to everyone (it's an MMO, after all) and making players feel like their actions have actually had an impact. I'm sure everyone and their brother has pointed out the resource nodes issue with phasing, so I won't beat that dead horse. The real trick is making it possible to help a friend with various quests even in phasing areas; as-is, you're just not going to be able to help a friend who's "behind" you in the phasing. They must catch up, and that's pretty well that.
So yeah, room for some improvements, but overall a good showing in my opinion. Plus, you somehow made vehicle quests suck a LITTLE less than they did in Wrath, even if they're still at best two-button spamming wonders.
The final point that I feel needs to be made is this:
Allow goofy-ass quest things to be repeated. Examples being Gnomebliteration in Uldum (hell, even let it give 150 Ramkahen rep if you're really feeling beneficent), and the "How I beat up Deathwing" guys in the Badlands.
Really neither of these needs any tangible reward attached, but just the opportunity to re-experience them without having to level new characters to those points would be fantastic. It's not like there aren't about a billion of those Gnomes, and the old fogeys in the Badlands actively tell you they have nothing better to do with their time than tell stories to people (I still want to see the knife fight happen =( ).