5. It will be crowded. VERY crowded.
You don't get 20 million annual visitors without the place getting super-crowded from time to time. Depending on the time of year—or even the time of day—when you visit, you may find the crowd levels perfectly manageable, or they may make the experience more of a nightmare. Go in knowing that it could be difficult to navigate the park, and that certain wait times will be an hour or more.