Spike damage.
Spread damage.
Spread debuffs (why do I still have an 8-second cooldown on my dispel?).
Overly punishing mechanical failures.
Random BIG HITS with no telegraph.
Also, healers have very poor catch-up mechanics in this expansion. If you get behind you are going to be FOREVER BEHIND.
When I run as Shadow I make a point to heal myself and my healer every time I have the mana, regardless of what it does to my DPS.