Kids are so different. I took my son on his 6th birthday. He was very excited to go on the HM, and was actually fine with everything UNTIL we got to the moving walkway to get into the Doombuggy, then he freaked out. Call me mean or whatever, but I dragged him on the ride. Turns out, he loved it, said it was his favorite ride and made us go on it again right away.
My 3 year old daughter would absolutely love it, she loves darker things. Her favorite Disney movie is Frankenweenie, and she also love Nightmare Before Christmas. I hope I can take her to
Disneyland this coming December, because although she has been on the NBC version of HM several times before, when she was a baby, she would actually be aware of it now.
With 3 years between them, my daughter isn't afraid of anything, but her big brother was too afraid to go on the Toy Story ride. I also couldn't get him to go on Space Mountain or the Matterhorn, or Splash Mountain. I almost didn't get him on Soaring, but after he went on it, of course that was the best ride ever. Oh, and he was afraid of meeting Winnie the Pooh. All of the other characters, he was very excited about, but something about Pooh freaks him out. I have a very strange child.