That particular stretch of road is not well-designed, IMO. Motorists headed to the MK resorts or to drop off at MK get confused in there with the lane changes when you exit the Toll Plaza, and they tend to stay confused all the way to the resorts unless they regularly drive in that area. The road is fairly narrow where you have to make that lane cross, and the number of busses getting bottlenecked there and again near Seven Seas is pretty high.
I agree that with the advent of Magical Express, resort capacity being what it is, the busses are starting to be overtaxed, especially when hours are shortened and you get more people funneling into one park at the same time. Light rail would be good, but the problem is the existence of the monorail -- tying it smoothly into another system that served the entire property would be difficult, as people who are unused to public transit tend to have issues with multiple transfers. I guess that next logical step will be articulated buses, but as Tyler once pointed out, the park stops are not made to accomodate them, and would have to be completely redesigned and rebuilt.