I can see how it's a nice idea (and liked the quests I've done where the town you're in suddenly comes under attack or stops being attacked after a quest), but like the abundance of cinematics (I got extremely angry with Uldum first time through) I don't like things which interupt my gameplay. Deepholm seemed really bad phasing wise on my first team and not being able to help friends is also a problem (they could at least add some sort of flag in for quests that you can't help people with because of phasing) and I had a quest where both my toons flew of to kill an npc but I didn't realise until halfway through that they both had their own version of the npc to attack.

Vehicle/escort quests where you only get credit for what the individual toon does (but both get credit if one stays out of the vehicle) and which remove focus once you're done (my second team was hardcoded instead of using focus). I can imagine these being a big pain for 5-man teams.

Cinematics that you either can't cancel out of or can cancel out of but won't have completed the quest if you do that. And I've had trouble with doing the quest on one toon, both having a completed message but only one of them has changed to the new phase.

The linear nature of questlines is also a bit dissapointing, removes any use/need for levelling guides but I imagine after a few times through each zone it will get really boring doing the same quests in the same sequence each time.

Lastly, whilst I really liked Vashj'ir as a zone I think it was about a third too long, I started my first duo there and felt like I would never get out.