That it took 12 years to get here and yet still so many races and classes to add.
The gaps in # of quests (and therefor xp) for some levels.
Not being able to join low epics while in high heroics.
Things look like they are generated on the fly but instead are there from the get go of the instance (and mobs not going away either.)
Getting video/engine lag due to slow loading / package loss.
Not being able to create your own quests.
The limitations of the inventory system, things being bound to character in particular.
Not being able to trade anything and everything, mostly where it involves ingredients.
After all this time we're still not able to do a mass raid, like 24+ peeps.
The low droprates, S/S/S system encourages multiboxing, overall it encourages cheating and buying things from outside the game,
The big gap between the leet haves and the noob haves not.