My ds (now 34 months old - almost the big 3) started going when he was 20 months old - and has already been back four more times since the original trip. Yeah, I know, a 3 year old who lives in Buffalo, NY and has been to disney 5 times already sounds a bit spoiled (he is), but I travel a lot for my job and I had a string of conferences(5 of them in 1 year) in Orlando so we just stayed at WDW. Anyway, we're going back again at the end of september as he LOVES it soo much.
So you ask, how do we know he loves it so much? About 2 weeks before our last time to visit the mouse, my ds picked up his back-pack, put a few toys in it and his yellow blanket, and walked to the front door and opened it. Now, he had never done that before. Anyway, we ask him where he's going. His answer, and I'll never forget it, was
"Disney world. Mickey's castle"
That should demonstrate how much he likes it - and remembers - at least for now.
Finally, I tend to agree that most kids will get a lot out of it if it's done properly... starting your kid watching disney channel early on helps with the introductions so to speak - but i digress i dont want to get yelled at for lettin him watch some tv. Don't rush things. Go back to the hotel for nap. Go swimming. Only do rides if the child wants to. Only spend time with the characters if the child wants to. Watch out for the heat if you go in summertime.