I guess I'm in the minority - but my kids all love Epcot.
My 3 yr. old daughter (now almost 4) loved Crush & the entire Living Seas experience. Usually preschoolers love aquariums because they can get up close to the most beautiful marine life around. She wouldn't leave that building!
My son (now 7) loved the little train setup outside of the cuckoo clock shop in the World Showcase. He still wants to see that everytime we go to World Showcase.
My oldest (now 12) loved the Figment ride when he was little. Now his brother & sister always want to go on it. They have a great indoor/interactive playground at the end.
Other "must do" for pre-schoolers:
The musical fountain show in the center hub - I don't know the schedule for that, but it is just beautiful.
The hedge maze in England / World Showcase.
Mexico pavilion indoor boat ride / World Showcase.
The Land boat ride / Future World.
I'm not sure of the height restrictions for Test Track but my boys both consider that #1 in Epcot. If he's not tall enough, then go in through the side entrance (to the gift shop) and there is a great room full of new model cars & concept cars that you can get in and out of. It is the exit area for the ride, where everone filters out into this awesome display for little guys (& big guys) who love cars.
Also, Epcot has a great character area that seems to never get really crowded. I don't know how publicized it is, but tons of characters are there, and they appear at sceduled times during the day.
Have Fun!