I will admit, Hellfire Peninsula is very cool looking, but everything else about BC sucks. There is no reason, beyond fun and nostalgia, to play anything high level that was in before BC.
That gets rid of:
Stratholme, Scholomance, Molten Core, Blackwing Lair, Naxxramas, Onyxia's Lair, Upper/Lower Blackrock Spire, AQ20, AQ40, the Feralas dungeon(I don't even remember what it's called, I didn't even know it existed until I had to do it for my epic mount THAT I DIDN'T GET.)
EDIT: Some clarification. While there is the fun and nostalgia factor here, I can assure you that NO ONE PLAYS ANY OF THESE DUNGEONS ANYMORE. I found one raid, it was just a 20-man though, and there were like five griefers just there to screw with people.
In my month or so in Outland, the only fun dungeon I played was the Underbog, because the bosses were all very cool looking. I've never finished a raid, and only participated in two(AQ20, we wiped on the final boss; and Grull's Lair, it was a Test Server, and it went down in the middle of fighting the first boss[es])
Looking at that screenshot of me and the giant gave me a big blast of nostalgia. I had so much fun before I got to Outland, and it took probably two weeks of Outland before I was having no fun at all, and just counting the days until I reached 70. When I hit a spot where I didn't have any completable quests, and would have to grind or do the boring, lame instances to get more quests, I couldn't handle the prospect, and just let my account run out. That screenshot is like a picture of a newlywed couple on their honeymoon, before they start fighting so loud the neighbors can hear.
But enough about my disillusionment. What class/race/level/talent build are you?