Celebration works better than Epcot... OK, that's a pretty wide open statement. Let me see if I can explain.
Urban planning concepts and architectural design from the late 1970s would not translate well into the real world of 2010. Look at the current state of Future World... Massive concrete structures that WDW is struggling to make look futuristic, or even current by adding neon, twirly wind things, new colors... The future has always been a problem for Disney, because it's a moving target. Design quickly becomes dated. It's a problem with Tomorrowland, so much so that they have redesigned it to show a vision of the future from the past, one old enough to be nostalgic.
With Future World, it's the same problem, but on a larger scale. With a full city like Epcot, the problem would have been massive. The center of the city would currently look like a shopping mall from the 70s, and would be requiring constant renovation. All of those fantastic transportation systems would also be requiring massive updates and maintenance, which makes the overhead of the operation really expensive, and results in a "city" that isn't sustainable from a cost or design standpoint.
Now consider Celebration. Instead of Tomorrowland, it's Main Street, with designs rooted in our collective past, private homes linked by walkways and paths and electric cars. No monorails to maintain, no massive central core of offices and apartments, no need to constantly update the architecture...
Compare Tomorrowland/Future World with Main Street. How much has Main Street been updated over the years? Except for some paint and the addition of retail space, not much.
In the end, we don't know what Walt would have thought. I get the idea that he was full of surprises. But I do know that just prior to his death, he was using the ceiling tiles over his bed to lay out the city of Epcot. Not a theme park. And I think the theme park, especially in its current state, would hack him off. "Future World? This is the future? Cars that go fast, time-traveling talk-show hosts, greenhouses and aquariums and a very dark imagination, a break-dancing space dude and an attraction that is mostly about the past, with the cartoon future playing out on a tiny TV for half of the ride? And a park full of people so interested in watching whatever is on their tiny phone screen that they can't be bothered to watch where they are going? WHERE'S MY FLYING CAR!!!"
In my imagination, I can see him walking through the city center of Celebration, commenting on how it reminds him of Marcelline, except for the fountain where the kids play, the bicycles and odd little vehicles scooting around, and the restaurants serving lunch outside, where you can overlook the pond and wondering where the alligators are.
And then Walt asks, "So where's the train station?"
Mick