In general I got the other way. Pick the things you must do and work around them. Everything else is a bonus.
I agree with this. It's going to be much easier to make a list of must-dos and then fill in with whatever time allows.
My daughter was 5 or 6 on our first trip, and we only tackled MK and Epcot. We intended to do all 4 parks, but we had so much fun at Epcot the first day - we wanted to go back the next one! Even so, we didn't see (and still haven't seen) all that those parks had to offer.
I love these boards, but if you don't already have one, I would recommend getting a good guide book (like Unofficial Guide). It will detail the attractions 1 by 1, tell you who likes them (little ones, teens, everyone), tell you the scare factor, etc. I would go through each park, attraction by attraction and highlight the ones you definitely want to see. It will probably be easier to start with EP, DHS or AK because they have less attractions. Then, if you have a question about one ride or another, come back here for answers. Don't get me wrong. I'll answer questions here all day long, but I think a good guide book is REALLY a valuable piece of the puzzle.
Also, not that you asked, but Epcot is my favorite park. If you're totally uninterested in World Showcase (and will only be touring Future World), then I would say a half day will do it, provided you get there early. However, if you're at all interested in World Showcase, a half day will be tough. By contrast, a half a day is more than enough time for my family at DHS and AK.
Edited to add: PP said to skip Journey into Imagination in Epcot. While I personally agree, it was my daughter's favorite ride up until last year!