I agree - that was just about the same age we took my son for the first time, and now this year will be his 5th trip and he's turning 5 this year. Looking back, that was probably the easiest and one of the most memorable trips we've ever taken. That was also the first time we ever went in early December (2001), and we had fantastic weather and no crowds. He loved the characters, and there's so many rides and shows they can do. One of my all time favorite photos of him is the first time he met one of the characters. I don't know who had a bigger grin on their face in the picture - him looking at Pluto, or me looking at him looking at Pluto.
I bought the "Walt Disney World with Kids" book by Kim Wiley I believe. I knew my way around, but liked reading about how the rides were rated on a scare factor for todlers.