I took my DS2.5 last year and he did fine. He didn't seem scared by much (if anything) and appreciated it at his own level. He fell asleep in his stroller, after taking a nap earlier in the day. We didn't last til the end, though.
there is not really anything truly scary at the mnsshp. my kids LOVE it, as do my DH and I. it is all very appropriate and nothing grotesque. plus, you get to trick or treat IN THE MAGIC KINGDOM!!!! if disney hands out anything for free, even if it is just candy, do not miss it!!!