Mainly, I think a good mentor is somebody who knows when it's time to put on the crown and when to take it off. You don't queue into Mentor Roulette if you're not ready to offer explanations or be patient. You also don't let yourself get spoiled by easy guildhest runs, and then pitch a fit if you happen to get Shiva EX.

IMO the most important quality a mentor should have is letting people make mistakes. Given how most of us are veterans of the current content, it's easy to want to bombard sprouts with mechanics or tips as soon as you load in. Unless someone asks, though, you really should let them experience the mechanics on their own for the first time.

There was a Garuda EX run I had where the other healer ditched after 4-5 wipes. Everybody else (I think 5 others were new) stayed, and I did my best to explain where we were failing while waiting on a new healer. Everybody got their clear on the next pull, and I think seeing their excitement outweighed the frustration.