I've taken my kids to WDW at the following ages (my kids are 26 months apart, so when the youngest was 12 months the oldest was 38 months):
12 months
18 months
24 months
38months
44 months
50 months
and we took our oldest when she was 22 months and the youngest was still in utero.
Here are the "top rides" for them
All of fantasyland, but especially
Winnie the Pooh
Small World
Dumbo
Also they liked:
Alladin's carpets
Country Bear Jamboree (my boy loved the country bears at ages 12 mo, 18 mo, and 24 mo - he wanted to watch them over and over and over each time we went)
Nemo (in Epcot)
3 Calleberos (Epcot)
Spaceship Earth (Epcot)
Triceratop Spin (animal kingdom)
Boneyard (AK - huge hit)
Petting Zoo (AK)
Disney Jr live on stage (HS - its totally worth the wait in line)
And the meet and greets. They LOVED the meet and greets. They even loved characters of movies they've never seen. I've waited to meet the Alice in Wonderland Rabbit even though they never saw the movie, the Aristocat cat even though they never saw the movie, etc.
Rides that are not a good idea:
Pirates of the Carribbean (nightmares about pirates)
Haunted Mansion (nightmares about ghosts)
Philharmagic/Laugh Floor/US Presidents/Muppet Show (won't keep 3d glasses on, can't sit still for 10+ minutes, can't follow the story or the jokes)
Tough to be a bug (too scary, can't sit still for 10 minutes, can't keep 3d glasses on)