Having done AD work on two decent budget indies for the past few years I would say the AD is what Scott & Clive said. I would say there is two roles an AD plays and these vary with the budget and shooting mentality of the production team.
What Scott & Clive said (managing the crew, cast, time sheets, call sheets, who has the food, etc.) all happen on films where the budget is big enough to have a heriarchy (sp?).
In an indie situation the AD is more of a Line Producer. Since they will have more to do with the setting up of the shoots then just the day to day operation. They will hire/fire crew, scout locations with the Dir./DP, etc. in addition to the above daily tasks. They do get stuck with the Boom alot too!
I agree it's a job with very little win-win situations, the job makes you look like an a-hole a lot. I was just talking with a friend of mine who did work on "The Island". He had never seen an AD who wasn't me. He found it funny that people would complain about the AD being an a-hole for just doing his job. He called me up the day they wrapped and told me that he finally understood why I was always the guy who after the shoot was buying drinks! My friend did say "The Island" AD didn't buy anyone drinks and yelled alot which isn't really cool since I never yell (except at people who can't turn off thier cell phones!).
I don't think I'm an expert on what makes a good AD but there are somethings I try to do to make everyone who works on the shoots time more enjoyable. Don't yell at people a lot! Sometimes you have to get a job done and it's gonna make someone else mad, if you don't make it personal you can get more accomplished. Always ask people to turn off thier cell phones, pdas, watches, etc. BEFORE sound is rolling! Remember each person on the crew believes thier job is the most important so if they come to you with an issue you need to help them (NOT ignore them) since they wouldn't come to you unless they felt the problem was going to affect the production. Make things very clear in your meetings with the crew about how you expect people to approach you. How you expect them to NOT approach certain people. How you feel the shoot is going to go. Always have a check up meeting first thing before the director, talent and producers arrive. Remember to smile your making a FILM not flipping burgers or fighting a war. Your job is very easy compared to some of the potentials out there.
I find it cool that several people mention that they always pay thier AD since I am one. Anyone hiring?