Do any of you have an actual policy? I tend to not get involved in PuGs at all if I think there's a chance they won't make it -- there are too many fish in the sea, and too many other things for me to work on rather than beating my head up against a boss. But maybe it would be good to set up a rule that most of us can agree on, something like, "three wipes and you're out." That might save a lot of time and frustration in the new Dungeon system.
Of course, the good news is that, on the opposite side of things, Blizzard has made it pretty easy to fill in spots for people who leave -- you just queue up again and get another player (though we'll have to see how people feel about signing up for an instance and appearing there only to find that two bosses are already down). But I'm sure quite a few groups will end up having problems, and when they do, and that stranger who's supposed to be healing decides to call it quits rather than continue on, just remember: they've had one too many wipes.