For our family, it's already a full-day park.
But, when I've traveled alone, I know that I'd want at least two additional attractions to keep me busy for the full day.
You see, when I go with my family (including my two little princesses), we MUST go to the Playhouse Disney: Live on Stage along with the Voyage to the Little Mermaid. Plus, the HSM and Beauty & The Beast shows. Those are two attractions and two shows that I would skip if running around the park solo or with another adult (or group of adults). Add in the fact that if my oldest daughter wasn't interested, I probably would not go to the Indy Stunt show or the Star Tours attractions.
This drops the list of "MUST-DO" attractions to....
Toy Story Mania, Great Movie Ride, Tower of Terror, Rock 'n' Roller Coaster, Action Stunt Show, Backlot Tour, MuppetVision and a couple of sitdown restaurants throughout the day until Fantasmic.
So, it's close, but not just quite the full-day experience for me on my own or a group of adult friends. But, it's definitely a full-day park for a family with children. IMO.