The first time we went our daughter was 3 years old and it was SO FUN! Here is the link to our trip report:
http://www.disboards.com/showthread.php?p=17369881#post17369881
I wrote about all the rides and her reaction to them. She was actually pretty scared in the dark rides at Fantasyland. Snow White, Mr Toad, and Pinocchio made her cry! She liked Peter Pan and Alice though. Buzz took some comvincig after the scary indoor rides, but once she was on, she loved it! Dumbo was her all time favorite! At DCA, she loved A Bug's Land...all the rides there are perfect. She also LOVED King Triton's carousel, she rode it 3 times in a row! The 3D movies were a bit scary for her though.
We did not take her on Haunted Mansion, I thought it was too scary for her. But she liked Pirates! She rode Splash Mountain (big time freak out!) I forgot about the first dip down and it totally freaked her out before even got to the big one! She wasn't too happy with me, but afterward she was fine (she didn't want to ride it again though!). She didn't want to ride Matterhorn, and I think she would've been frightened by the dark & the snow monster. She rode Thunder Mountain, but didn't want to ride it again.
She loved Tarzan's Treehouse, it was really fun! Also, the Sailing Ship Columbia, the Train, and Tom Sawyer's Island are must-do's. Lots of things to see and do, and its a nice break from standing in line.
We also had to take nap breaks around noon every day for a couple hours or she got CRANKY!!! And by day three, she was done. Two days would've been fine. Three was pushing it, we didn't even stay past lunch.
Definitely bring a stroller, snacks, water. And if you need anything, the first aid station and Baby's Lounge are awesome!