To be fair, hardly anyone visits all four Disney World parks in a single day. The Disney cartel recommends that families stay a week, so that the maximum fun and cash can be extracted by the two sides. But with the new one-day "park hopper" ticket that Disney World now offers, it's possible to cram the whole joint -- Animal Kingdom, Epcot, Disney-MGM Studios and Magic Kingdom -- into a single day's visit.
No one is saying it can't be done, but this is one way to kill oneself and not enjoy anything about the parks. It will be even worse if you have young kids.