I quit WoW some time ago, but I still occassionally check what's going on in Azeroth. Recent Post Mortem interviews (especially the one with GC) made me really interested with MoP. I think it may cure some of the crap taste that Cata left in many people's mouths.
My favorite thing in WoW is leveling your toons. It's like an epic journey, meeting new people, visiting new and old places, listening to the exceptional WoW soundtrack and many, many other things. My biggest concern with MoP is the 1-80 leveling process. Blizzard has been reducing the amount of experience to level up gradually over the expansion packs, and I think Cata is where they really crossed the line.
The low-level questing content just doesn't go in par with the current experience system and it feels too fast and rushed overall. All of the low-level areas have been revamped in Cata and it pains me that I can't fully experience their content, because I just keep outleveling everything. I always follow the questlines, never grind, do instances just one time to get the achievement and complete quests associated with them, and I still outlevel every single area I come to. And I don't mean just some slight overleveling. After some time I find ONLY grey quests in my questlog, which is disturbing. I talked with a lot of my friends about this concern, and they agree that something is off. This thing happens to people even when they attempt to minimize the experience flow by avoiding killing unnecessary NPCs.
A lot of players (including me) like to complete all quests in a zone before leaving it and then proceed to the next one. You can't really do that in Cata, because you will find yourself surrounded by grey quests.
I understand that Blizzard wants to make the leveling process slower, so everyone can start raiding and get them shiny epix as soon as possible, but please don't forget about people who play not only for endgame content, but also for leveling your toon (yes, there are such people). Fix this and make low-level questing a longer, more epic adventure, considering all the hard work you did revamping the old zones, so that players can actually experience them instead of just rushing through the content, skipping most of the quests.
I believe there are ways to make all people happy. Make the leveling process longer (but not EXTREMELY longer), so that your level actually flows with the area you're in.