"Specific" Random Dungeon Finder
My suggestion is that the first RDF of the day for each players (depending on whether the GS bracket allows it or not), should be a "specific-random" for everybody, similarly to the "Must Die" quests, but invisible to the player. The reason behind that is to trample a bit more on the people who enter a dungeon and randomly POOF disconnect whenever the dungeon is Halls of Reflection, The Culling of Stratholme or The Occulus, or even leave the dungeon in that matter and taking the 30 minute debuff, in that matter.
Here's a hypothetical.
Johny enters a CoS and ALT + F4's or leaves. Johny is a tank & Johny's group has to wait 15 minutes more to be able to start, after waiting 15 minutes by then already. Johny, on the other hand, has another toon he logs and instantly queues for a different dungeon, starting a 15 minute wait timer for himself while inconveniencing others by, what is by then, half an hour.
With this system I am proposing, Johny will be discouraged to leave the group at all, because it will be KNOWN that the dungeon he is going to enter with his next character will AGAIN be CoS and that there's no avoiding it, other than not doing an RDF at all.