You know what? People are talking about removing the Roulette rewards for dungeons after a certain number of times that this happens, and I think that is an even better solution to the DF cooldown timer. Because someone who gets the same or similar dungeon they do not want run a half hour later may not stay in that party either. But if you remove the reason for the Roulette altogether (the bonus tomestomes and/or XP) then people will either NOT queue and waste their time and others' time, or they will queue and just do what they should have in the first place--finish the dungeon.
I also believe the penalty should be harsher if someone leaves in the first two minutes OR without clearing the first room/hall. If something comes up that quickly that is legitimate, then you probably won't be available to queue back up so soon anyhow. A high penalty will not affect you, but it WILL impact that abandons their group without trying to make it work.
