I've used a lock since the start. Before BC. I've noticed one thing about locks that really annoys me. If my demon kills something without me dealing damage to the mob myself, I get no drop. The reason this is annoying is when the mobs are weak.
A good example of this annoyance would be running someone through a low lvl instance. I could go demo spec and have my FG 1-2 hit kill the mobs easily, but if I do that, no drops for anyone. Due to the low lvl of the mobs, it's a 1 shot kill with nearly any spell used. That's fine, but uses lots of mana to kill hundreds of simple mobs.
I just wish pet solo kills were loot-able.