Reader Poga dropped us a line to this article he wrote for his guild about the "ten commandments of being in a group" (perhaps inspired by my ten commandments of dueling). Pretty much every group problem is in here, from stealing aggro to breaking CC to loot whoring.
The pulling one is an especially good tip-- decide at the beginning of the run who's going to pull, and then only have that person pull. Can't tell you how many times, someone decided to just pull with an instant spell because they felt like it, and not realizing that another pull was already incoming. And I think the "not wear thy gear in vain" tip is an interesting one, too-- you should putting gems and enchantments on your gear already, just to make your character the best it can be. Seeing it as a way of benefiting the group is a new way of looking at it, but it's true as well.
If everyone followed these tips all the time (and even the best tanks I know sometimes forget to wait for mana), we'd all have better groups, whether they be PuGs, good friends, or raids.