We just got back from our second trip the WDW. Last year my then 5yo and 2.5yo rode loved and rode everything they were tall enough for. We took them on everything without telling them what would be scary/dark. I didn't put any preconceived notions in their heads. HM and Dinosaur were not tops on their lists, but they didn't cry or freak out. When we got back I bought the Happiest Celebration on Earth CD and they have listened to it A LOT, to get them familiar with the rides. When we went back this year, my now 3.5yo was tall enough for TT and ToT. We did Dinosaur again with them saying they wanted to ride (still not a big hit) and they loved HM, maybe because they were now familiar with it through the music. TT was a hit with DD3, but she didn't like ToT. No crying but just said she didn't like it and didn't want to ride again. I think it all depends on your kid and how you present the ride to them.
With that said, some of the favorites of my 3yo are Teacups, Carousel, Buzz. On the other hand, with her liking HM and PotC, she is scared to death of the costumed characters. Go figure.