Last time we went one DS was 15 months. What he enjoyed was...
MK: Peter Pan, Dumbo, the MK RR, Donald' Boat, Playground in ToonTown, Playground by Splash Mountain, The Carasoul (so short though, I don't recommend it unless you can just walk on again and again), Aladin's Magic Carpets, Pooh.
Epcot: the water fountains, watching street shows, Journey into the Imagination
AK: Festival of The Lion King, It's Tough to Be a Bug, Kilamajaro Safari, Bone Yard, TRiceratop Spin
MGM: Playhouse Disney!
He also really enjoyed parades everywhere and the fireworks. And monorail and boat rides.
THings he did not like were characters in the first half of the week unless at a distance. He did warm up to them though, except the face characters--never did like the ones that spoke to him.
There were many other things he was just luke warm to.
I want to add that this is a VERY high maintance child, and at WDW, he was pretty easy. Our older DS was never any harder at WDW than at home either. I don't think you'll have a problem at all taking a toddler to WDW. Just be sure to bring some favorite snacks/foods, plenty of diapers (2X what you think you'll need just in case) and an extra change of clothes.