Sound quality, comfort, build quality. I need to not feel that there is a headset there. Also, I need to not hear the cord rub against things when I move my head, drives me bonkers.
Ergonomics is first for me. Which not only includes comfort but ease of button use. My usual gripe with gaming mice is the terrible location for the sensitivity buttons. A game like Tribes can require you to constantly switch between sensitivities if you're playing as a sniper.