In the MK, I like the chicken salad without chicken at Pecos Bill's (the veggie option is listed on the menu right below the chicken salad and sells for a couple of dollars less). It has jicama, lettuce, vegetables and cheese in a lime vinaigrette.
Also in the MK, Columbia Harbor House has good vegetarian chili.
I second the vote for the Vegetarian platter at Tangerine Cafe in Epcot.
I don't remember anything remarkable at the other two parks, but it's been a couple of years since I've been there.