Well, we recently went when my daughter was 11 months. I was lucky in the fact that she liked to sleep a lot and her temperment was pretty mild.She wasn't walking yet, so that helped. If I took her now at 18mth I think I'd have to be heavily sedated. It's a whole new ballgame when they know they can get out of the stroller.
Anyhow, the trip went pretty smooth, we just had to slow it down. I took a stroller that had a full recline which saved us from having to leave a lot to go back for naps. We were able to take her on most rides amazingly enough. Not, of course on anything like space mountain, but a lot more than I had thought. You can take advantage of the "parent swap" system if you don't have an extra adult to watch the little ones. (It's like a fast pass for parents).
To help with the naps, I brought a smaller light weight blanket to drape over the front of the stroller canopy and secured it to the top with 2 clothespins. That way I could just flip it up when she was awake and down when she needed a nap. I also brought along a stroller fan which I never had to actually use since the weather was perfect with low humidity.
To help with my travel load since we were flying, before we left we ordered stuff from garden grocer (babyfood, diapers, formula, snacks, etc...) and had it sent to our hotel. We used a pack-n-play from Disney.
So all and all, it wasn't bad. You just have to prepare yourself mentally that you won't be able to "do it all" when you travel with a child that young. Also, you know your child's temperment best, so that might help you gage how your experience will go.
PS- We stayed at Coronado Springs Resort and All Star Movies. CSR was wonderful, and the bus system better than Movies. But my son really loved staying in the Toy Story section at ASMo. He still talks about it 6 months later. My first choice travelling with a baby would have been Contemporary, GF, or the Poly since they're on the monorail system. But they weren't in our budget at the time.