...

Someone (Neil?) raised the question of description in fanfic. Do people prefer people and backgrounds to be described, and in how much detail? How overt does this have to be before it gets annoying? Does anyone have any useful hints?

I would advise looking at some of your favorite works of professional fic and see if there's a style you'd like to emulate. A paragraph is generally plenty to give us a picture of an important character or setting, with a few extra details perhaps dropped in later on. Minor characters or backgrounds should be described in passing:

"As Vila dodged past the guard on the wrong side of the rubbish bins without being seen, he noticed the uniform folded oddly and did a double-take. He'd never noticed a girl in the ranks before. Well, couldn't warn her about the bombs without giving the game away." At this point, if she's to become a player in the game, she might stop Vila, or might take off her helmet to deal with an itchy ear. If not, Vila's thoughts should turn to how much time he has to rendezvous with the others.