I wouldn't like to tarnish every tank with the same brush, because I have had a literal two amazing tanks, I am also sure there are many other tanks out there that are also good at there chosen class. However the rest of the tanks I have had in dungeons have been absolutely shocking.
I had one that point blank refused to accepted that I had three adds hitting me which killed me. He said he "had agro" on everything because he checked the "traffic light system", apparently meaning I died to thin air and those three mobs were a figment of my imagination. Since then I have been a bit more forward with making sure the tank picks up adds.