NPCs spawning from nowhere, allies constantly getting in the way of your fireballs (if you're a destruction mage). About 50% of the enemy NPCs are invisible; this glitch seems to occur when you run up ahead of your allies. So if you try to get the encounter over with you have to deal with invisible/invulnerable NPCs that are able to attack you.
Not only that, but the whole idea just seems boring to begin with. Even if there weren't allies getting caught in my spells and invisible enemies, the task of killing a certain number of enemy NPCs that are spawning from nowhere is tedious and boring. This feels like a World of Warcraft quest. If you had to do this once, twice, maybe even three times, then it wouldn't be so bad. But you have to do this several times.