Myth: Staying on-property means you are only a few minutes away from any WDW park.
Fact: While I believe staying on-property is far more convenient than off-property, the fact is, between waiting for transportation, transit time (including stopping at other resorts), and getting through the gates, many times it takes the better part of an hour to get from your WDW resort to the inside of a park.
While I think WDW transportation is efficient, I'm still amazed at the amount of people who think skipping from park-to-park or resort-to-resort will only take a few minutes. Granted, some resorts are conveniently located next to specific parks (monorail resorts & MK, boardwalk resorts & EPCOT), but the vast majority of the time, park/resort hopping will eat up some big chunks of time (especially if you don't have a very good plan).