On the other end of the spectrum, on my healer I get most annoyed when the tank and DPS are not helping me at all, all attacking the same elite mob while 4 normals are shooting me from range nonstop. And then they go "NEED HEALS" even though I'm having to spam heal myself just to stay alive...
This is perhaps the most annoying thing when playing as a healer. I have quit after the team wiped repeatedly and blamed me for it (because it's always the healer's fault when things go wrong). What was actually happening is what you described - the three other players (a guardian and two sentinels) leaped in and all attacked the boss. The adds and other beasties were ignored, and when they that happens they always
attack the healer. Then the others screamed abuse at me because I was spending all my heals on myself.
I considered lecturing them (especially the tunnel-vision sentinels, one of whom kept using Guarded by the Force even when he was at full health), but then thought it wasn't worth it and quit. My one and only rage quit. Most of the time I'm happy to go on even in the face of repeated wipes (there is in fact no better feeling in the game than when you finally beat a foe that had been kicking your butt repeatedly), but being blamed for other people's mistakes was just too much.