I'd also like to say that out of the tanks I do get in the DF, most are just using it to get faster groups. And as such tend to be complete (insert derogatory term here) to anyone they come across, and are focused purely on speed running even if the other players actively tell them that they're new to the run. They're especially brutal to healers that aren't "good enough" too keep them alive when they're pulling groups that are large enough that they do as much damage in two hits as the tanks have total health. My guess is that this is a large part of why so many DPS and Healers are hard on the rare tanks that aren't like that, as most players using the DF will have had more bad experiences with tanks than good ones. This in turn creates the situation that makes people not want to tank in the DF except for those who are just in it for the quick run, and the cycle continues.......

EDIT: I would like to say that I'm just judging this based on my experience, and seeing how people react to tanks in such experience. I heal and DPS mostly, and have only tanked low level Guildhests at the most, so I'm purely talking from a "bystander" perspective.....