While you can't see all the MK has to offer in one day, you should be able to see most of what you want. First, determine if you are going during a peak or slow time and what the park hours will be. Identify the rides, attractions, and shows that you want to see. Check to see which ones have fastpass. It also helps to know which rides and shows are long and which are short. Skip the parade and go on rides instead since they tend to be slower during parades. Develope a touring plan based upon this.
The key is to minimize standing in long lines. If a ride has a really long line, get a FP. If you can't get a FP, consider skipping the ride and moving on to something else.
With some advance planning and the use of FP, you can see an amazing amount in a single day.