It's amazing to me exactly how hard instances can be. Not the mobs, but the group. We've all been in ugh-PUGs before. Some players don't understand how to use their class or play nicely in an instance. That's only on regular dungeons; heroics have their own set of potential disasters. Sometimes you just have to give up.
Putting a group together for instance can be extremely difficult, and sometimes you have to take what you can get. While there are many excellent tanks out there, the shortage on my server makes for slim pickings. Folks who think that pulling aggro off the tank is an honor make up another problem. I am occasionally guilty of trying to heal and DPS at the same time. I'll admit to causing a wipe or two because of it.