How about.. hmm.. everytime you log in, you go into Tank and need to do one Dungeon to unlock your healer and dps?

Everyone would Tank, no one would complain that Tank doesn't know what they are doing because.. they would tank themselves.

The Queues will definitely be faster, as everyone would like to get rid of this annoying thing in order to play the role they want for the rest of their session.

They would make sure not to pull too many, as a wipe even once means more time spent in Tank (oh noes!)

Kills the freedom of choice (I play whatever I want so get the hell away), but wouldn't that resolve the queue problems, albeit a bit?