Big ones in approximate order of how often they seem to occur (with my own definitions of flavor descriptors):
1) DMS (creamed corn, cabbage, celery, rotten vegetables)
2) Hot alcohol / solvent (bad vodka)
3) Oxidation (wet cardboard & sadness)
4) Diacetyl (butter or butterscotch or slickness)
5) Extract twang (metallic caramel & hint of banana)
I'm somewhat sensitive to DMS, so I might pick on it more than it is perceived by others. I don't usually pick it up in my own beers. I think it's due to less than adequate boil rigor primarily, and I boil every batch super vigorously.
Solvent flavors basically come from fermenting too hot. Keep it cool, eh?!
There are different forms of oxidation, but the stale form will happen to any/every beer with enough age, so it's always a possibility.
The other stuff really doesn't happen as much anymore in my experience. Even diacetyl isn't as prevalent as much anymore, and extract twang is less and less prevalent, at least with brewers who've got a couple years experience and know to use fresh dry extract.
By the way... a personal peeve of mine.... astringency is WAY less prevalent than most judges will tell you. I would say that 3 times out of 4 that a judge uses the term "slight astringency", they are in fact full of crap, trying to show off their judging prowess or something. This occurs greatly with inexperienced judges but unfortunately often continues farther up the ranks. While astringency is indeed very possible, I've experienced it many times, the term is WAY overused. People describe it like a bitterness in the flavor. I've even seen a Master judge use the term when describing the aroma! Totally, totally wrong. It's a dryness, as if you're sucking on a sponge or have been mouth-breathing in the Mojave Desert. Can best be duplicated by chewing a while on grape skins, after the juice is all gone, just keep chewing on those skins. That dryness, which is also a sort of spiciness, is astringency, and it's usually caused either by pH problems or by wild critters. It's not the same as bitterness. Be cognizant of this common error.