Just to add, I think that what they are adding to the park will make it a full day park if you experience all of it. I went last week with a friend who was visiting for the first time so we did everything the park had to offer except for Finding Nemo the Musical and Rafikis Planet Watch. We had booked three fast passes before hand for Safari, Everest and Kali Rapids. We were there from 10:30-6 and got everything done with very little waits on all the attractions and shows and had lunch. Now if you add in being there when its a little busier, doing the two things we missed, plus Avatar, Rivers of Light, the night safari as a second time riding to see both versions in one day and having dinner you definitely would get a whole days worth.
The problem for most I think is that they run from ride to ride, don't bother with walking trails or the animals and they only ride the handful of big attractions. Sure if you still go and only ride Dinosaur, Everest, Safari and Avatar, your gonna be in and out of there in no time. But if your spending the time to do Flights of Wonder, Bugs Life, Kali and the walking trails you can really make a day of it.
Honestly, finally slowing down at this park and just taking the time to relax and just observe the animals and do some of the smaller things, I have to say it was one of the best days I've ever had at the park.