@ Sleutel

The main problem with an "Unprepared for Role" vote that you've described is that it could, and most likely WOULD, be heavily abused.

OMG! That DPS is only doing 1500 DPS in a heroic compared to your 4k DPS! That must mean they suck and don't deserve to play! (As opposed to simply being a fresh 80 trying to gear up.)

or, That tank is doing well enough, but I don't like his attitude. Since most of the group are my buddies, I'm going to vote to make it so he can't run dungeons for 24 hours!

or, Gee, I'm an asshole who likes to piss people off for giggles. Let's see how many people my buddies and I can pull into our random group, vote to ban from dungeons, then boot!

As you can see, the abuse could be horrendous. Not to mention, the punishment wouldn't be helpful to anyone in the long run. ESPECIALLY for people simply trying to gear up.

The best way to punish people who queue up as tank/healer with no intention of playing that roll is for them to constantly be booted from their group. If no one is willing to do the job they signed up for FOR them, it'll eventually become apparent that that particular scheme isn't worthwhile, and they'll queue as the roll they want to play. Afterall, 30 minutes of queues is better than 3 hours of constantly being booted from dungeons because you have no intention of signing up for a roll you can pull off.