DS is 2, and what scares him/doesn't scare him varies by trip (we are passholders).
He's so-so with Pirates, but he will ride it. He really doesn't like the first battle scene with the cannons, and I prepare him ahead of time for the drop at the beginning ("okay, we are going to go down a little hill like a rollercoaster... get ready to go wheee!").
I was surprised that he was okay on the Haunted Mansion (took him right around his 2nd birthday), but I think that is because he loves the
Disneyland Fun Sing-a-long video, which shows some of the scenes.
When he seems unsure on rides, I whisper with him and make it more fun/comfortable.
Out of all the rides, the past few trips he has flat out refused to go on the Nemo ride at Epcot. Fireworks also freak him out.
He also once clung to me with his face in my shoulder throughout most of Journey into Imagination... and then said "that was a fun ride!" when it was over.
Two year olds... Just follow their lead...