Let's start by saying the notion of whether or not Animal Kingdom is a "half-day", "full-day", or even "two-day" park is nonsense, but since people seem to persist in discussing this topic I feel justified in weighing in (with knowledge that things will completely change in 2017 when the new nighttime enhancements and Avatar land are fully operational). So, why is this nonsense? The number of days one spends in a particular Walt Disney World theme park is directly related to the number of days that one is away on vacation. If you are staying at a Disney resort (or nearby) for two weeks, then you're bound to spend at least one full day at each park (and likely, more than one), whereas, if you're only in town for a three-day weekend, then you're not likely to spend an entire day, and certainly not two full days, in Disney's Animal Kingdom. So, the question isn't whether there is enough to do in the park to fill up a day if you had to (there's more than enough in any Disney park to do that), but rather how much of your limited vacation time you wish to spend in Animal Kingdom versus elsewhere on property (since I'm assuming you can't magically make the time or money for an extra vacation day appear).
For the purposes of this discussion, let's work under the assumption for now that you have five full days in the Disney parks. That means you won't see everything in every park and thus, you will have to make some difficult decisions about where to spend your time. If you want to take an evening to see Hoop Dee Doo Review or La Nouba, to explore Downtown Disney, to play miniature golf, to tour some of the resorts, or to spend a morning in one of the water parks, then that time comes at the expense of some other potentially enjoyable activity. Most Disney World visitors are limited in the time they can devote to their visits and the money they can spend, so adding another day just because there are things they could do to fill it is not an option. Thus, if you wish to see more of Disney World than just the theme parks, then you have to carve out park time somewhere. As for those five days, let's start with a full day in each park and let's devote a second full day to the Magic Kingdom. Now, where are we going to find time for those extra activities? My suggestion is that Animal Kingdom is the first place to start. Here's why.
Outside of a couple attractive views Disney's Animal Kingdom is not at its best in the evening. Couple that with early closings and it's easy to justify using your Animal Kingdom day as one where you spend your night elsewhere on property. Some of what makes Animal Kingdom most interesting - the safari, the animal trails, and the little details carved into the Tree of Life and elsewhere on property - are not visible (or hardly visible) after the sun goes down. So, while parts of the park are dramatically lit and rather impressive looking at night, most visitors eventually want to find something to do. It's not much fun to walk along an animal viewing trail when it's too dark to see the animals and it's not a very natural environment to put spotlights on those creatures just for our enjoyment. The lack of a nighttime show hurts too, as I'm more likely to want to end my day in Epcot or the Magic Kingdom because I enjoy Illuminations and Wishes (I don't feel as strongly about Fantasmic! at the Studios, but that is a draw for many). Disney has acknowledged this as they are developing a river light show for Animal Kingdom, which is likely the first step in transforming the park into a true full-day destination.
Animal Kingdom is also lacking must-do dinner options, which bring me back to the other parks at night. Dining in Epcot is a big reason that I like park-hopping. There are so many great dining options there, both counter and table service, that I can't fit in everything I want to eat merely around the attractions in Epcot, so I'll often "hop" there for dinner after visiting another park in the morning. Were I to spend a full day in Animal Kingdom I could make do with decent dining options in the park, but I would be happier eating in Epcot. So, what if you're not a park-hopper? I'm also a big fan of dining at the resorts and there are enough restaurants within a short monorail or boat ride of the Magic Kingdom that I could fill my whole vacation eating dinner at favorite places there, and I don't have to use a park admission day to access any of those locations. When I have traveled to Animal Kingdom with friends who don't "hop", we've often used the Animal Kingdom evening for dining near the Magic Kingdom, then watching fireworks from one of the resorts. I've also visited in December, when we used Animal Kingdom evening to see Christmas decorations around the Magic Kingdom resorts. I enjoy Flights of Wonder and the Finding Nemo musical, but if it means skipping one or both of those shows to see the Grand Floridian and/or Wilderness Lodge Christmas trees, dine at 'Ohana, and watch Holiday Wishes, then I'm choosing the latter every time.
So, let's say you're with me so far, but why not sacrifice time in Disney's Hollywood Studios instead of Animal Kingdom? That's a fair question, particularly as construction walls become the biggest attraction in that park for the next few years. The answer is that I will often limit my Hollywood Studios time to one or two mornings, so I wouldn't argue if you also described that destination as it stands today as a "half-day park". There are several differences, however, starting with re-rideability. Star Tours, Rock n Roller Coaster, and Tower of Terror are all attractions that I'm happy to ride more than once in the same day, and I'd say the same thing about Toy Story Midway Mania were it practical to do so without obscenely long lines. In Animal Kingdom, I could say the same about Everest, but that's about it. The Safari could be interesting at different times of day, but it's a lot of time to devote to the hope that you may catch the rare time when the lions aren't sleeping. Next is dining, where the table service options at the Studios offer a little better quality and are a little more distinctively Disney. Brown Derby offers better food that anything in Animal Kingdom, while 50's Prime Time Café and the Sci-Fi Dine In Theater offer unique dining experiences. In addition to attractions and dining, the Studios also has the edge in nighttime activity, particularly in the wintertime. With Fantasmic! running every night and the Osborne Lights, there are more reasons to return to the Studios in the evenings than there are to visit Animal Kingdom. Add to that special events, like Star Wars Weekends or holiday fireworks and there are often even more evening activities. All that considered, if I have to choose between the two parks, then I'm more likely to spend a full day at Disney's Hollywood Studios than the Animal Kingdom, but I'm not averse to spending a half-day in both parks during my Disney World visits.
Disney's Animal Kingdom is a great theme park and zoo to visit (it also IS a zoo, by the way, which perhaps I'll cover another time), but given anything but a very long vacation there will likely be other activities in your Disney World vacation that will take priority over spending a full day exploring the park. Like every Disney theme park, there are details that are only appreciated after many visits and there is always more to see each time you return, but since your vacation time is fixed it is hard to justify devoting more than a portion of a day to the Animal Kingdom. The park hours are too short, there are fewer must-do dining options than in Epcot or around the Magic Kingdom, and there (as of now) no nighttime entertainment to cap off your day. It's pretty clear to me why Animal Kingdom as it stands today remains a half-day park for most Disney World guests.