The expansion is really really good, best expansion since wrath for me so far. My only gripe is the class balance which is a complete ass. I know blizzard was always shit at balancing but I feel this time they went a bit overboard. Some classes feel godlike (dh, rogues) while other classes and specs are worthless (aff, demo, palas) you would expect them after 12 years working on the same game they would have learnt by now a trick or two about balancing but nope, as bad as ever.
I think their mistake is that when blizzard buffs or nerfs they do it with big percentages. Like in 6.2 demo locks were nerfed by 25% across the board and during 5.1 mage's combustion was nerfed by 50%!!!! (that is when I quit the class because I had enough). When you nerf something by those big numbers you don't just nerf a spec you bring it to the ground, you break it and make it uncompetitive. I love how Icefrog does it on dota 2. You always see tiny changes of 1-2% that slowly bring the heroes where he wants them to be and then leaves them there. Not a fucking 25% nerf, that is like deleting the spec from the game. Doubt blizz will ever learn though.