So I just stumbled into the 'Flame Temple Tombs' on live servers. I riddled my ass off for an hour and came to the second to last room. I lacked an item I would've gotten right at the beginning of the dungeon, but I never went there. Eventually I gave up and checked it out on youtube.
Btw. you totally can collapse that floor in that room he thinks is pointless, they're just progressing ass-backwards in the beginning. Anyways - this dungeoneering experience has blown me away, and I don't want to spoil it for people who already play the game.
 
For those of you still in need of persuading... watch it, and be amazed!
 
  

When I did this with a zerg, I was the sword bearer the whole way through. We took the collapsing route and made it all the way to the boss. After we defeated him everyone in the room spawned a chest of loot. Since each chest was instanced and there were about 15 of us, I ended up with massive quantities of loot. It was glorious.

Yeah, man, I ran through this place with a small team of three at about four levels lower than the generic mobs. We scraped our way through (at one point the other two guys had to go back for the sword while I stayed to open the gate for them), and even with a last-second addition of another two players, we only managed to beat that fucking savage final boss because he glitched into a wall. Indoor combat is hard in this game.

Just ran into this myself, it was pretty awesome especially since I was basically just wandering around randomly when I found it. Hopefully other zones have cool little mini dungeons like this to find.