Also, in regards to IngeKy's post, "dungeons that last 15-30 minutes" cannot possibly equal "pretty difficult", no matter what.
If you can do it in less than half an hour, its a no-brainer. Either its extremely easy to stomp everything on the way to the goal, or the schematic for how to avoid all the nasty bits has been worked out and one dude streaks to the end and teleports everybody else. Twenty minutes later, everybody jumps through the "I won!" portal that opens and you're back in town, sloshing back a cold one.
Although now that I think about it - want a hard, but fast dungeon? ***** the fighting through the trash mobs and navigating tricky mazes. Walk in, there He is, the big, fat, nasty dragon. Everybody pulls out the big guns and immediately starts DPS rotations while the CC guys attempt to block his 50' tail from doing a Massacre Sweep by stunning it so it won't move.
30 minutes later, everybody is back in town and all is well - but for those 1800 seconds previous, you were balls to the wall fighting with everything you had, without space to breathe. Lose the time wasting content that dungeons are currently full of.