I get upset when speed running is done with newbies around or if someone says "smaller pulls, please" and someone still runs off. Another time is when tank literally just pulls a bunch, stays long enough for DPS to start hitting them to get aggro, then leaves the group with a bunch of mobs and I get all the aggro because I heal. And I'm not talking about new tanks, I talk about those jerks on level 50 or so that just don't care in lower level dungeons.

As a healer I can do my job most of the time (sometimes even when I thought I'd never get through it and have desperatedly healed everyone to the point where i'm out of MP seconds before mobs are dead), but if I see someone else is having trouble, I will ask for smaller pulls, just like I will say bigger ones is okay for me if I see everyone is doing well.