As the title says, I am interested to hear your reasons for liking Ulduar. It consistently shows up in almost everyone's list of favorite raids.
I am probably biased because I was in a shitty raid guild when it launched and didn't transfer until after ToC was released, so by the time I got to do any serious raiding in there it was already outdated. I like a lot of things about the instance... it's huge, lot of bosses, interesting mechanics, organic hard modes. For whatever reason it just doesn't get me going, though.