We used to run Zul'Gurub weekly, if not more often. The loot was OK (a couple of GREAT tanking swords drop there, and not a lot else), the reputation rewards were fantastic, as were the enchants. Since Burning Crusade, we haven't even gone to Stranglethorn Vale. Is it less fun? I'm not sure. With the boss scaling issue, and trying to get everyone up to level, we just haven't taken the time to check it out. Since the shoulder enchants and head slot enchant are still very good, I may try to get a group to go back on a bijou and coin farming mission. We might just go back to see if we can run the place with 5 people for kicks.
That said, if I never set foot into Molten Core again, I'll be happy. I'd like to finish Blackwing Lair and poke into Naxxramas since I never managed to get there, but the new 5-man instances and new raids are keeping me adequately busy for now.
Any plans to go back in time now with new and improved gear and abilities?