Not enough quests and those that were there at times were in insane locations with no way of knowing they were there. If there had been enough of them for leveling I'd not have cared and just found those weird location quest givers a good thing. Then they would have been an actual exploration bonus.
Bad quests: A lot of them were downright boring. Effectivly, there were three types: Kill counter, drop counter and escort.
Bad quests: Insanity in where you had to go for quests, they often felt less 'there is a reason for going here' and more like 'they threw a dart on the map for where you should go'.
Loot stats: Way way to many items you just had to toss as their stats were useless. Including quest rewards.
Specs: Some just did not work well as options to pick in the trees. Not for questing, not for raiding. Moonkin being one that quickly comes to mind. Retri paladins were hard to take down, but were insanely slow to quest with.
Server stability: This was just bad, luckily for classic this should be way less an issue.