What do you do with these players?
I thought people were exaggerating when they told stories about DPS running about with swords and shields using Puncture on everything... but I just ran a couple PUG dungeons where exactly that happened. I asked one of them if he wanted to tank instead; he declined, but continued taunting everything. These pseudo-tanks were doing unhelpful stuff like pulling Sellistrix into the water and turning her to face the healer. The other DPS and the healer in both cases were solid, but if this had been a less experienced group, the dungeon would have fallen apart quickly.
Perhaps part of it is simply that they don't know what a taunt is or don't recognise that some skills taunt. It might help to make the tooltips on these skills clearer (e.g. red text that it's a tanking skill).
I would also love it if the game offered a really basic tutorial on roles that players had to do before queuing for dungeons.