Different vegetables require different nutrients - or different proportions of nutrients. These can roughly be grouped by families. So, cabbage, brusels sprouts, and cauliflower, all being of the same family, have about the same effect on the soil. Potatoes, tomatoes, and peppers likewise.
If I plant the same group of vegetables in the same spot year after year, that patch of soil would eventually be depleted of the nutrients needed by that group. Pest problems would probably also worsen each year, since the pests specific to that group could just stay put and wait for the new round of crops in the spring. These are two major reasons for crop rotation.
As I am putting my plan together, I am trying to use only vegetables of the same family within each bed. It's not a totally rigid rule, but will simplify the process of crop rotation in the future - next year, I will be able to keep the grouping of vegetables within the bed the same, just put them in a different bed that I did the previous year.
It does become a little more complicated than this. For example, lettuce loves nitrogen and beans replete soil nitrogen, so it makes sense for lettuce to follow beans. Other plants (such as broccoli, I believe) will produce all leaves and no fruit if there is too much nitrogen in the soil. Therefore, in one bed, it makes sense to plant beans one year to replete the nitrogen stores, lettuce the next year to thrive on that nitrogen, then broccoli the following year once the nitrogen level has been tempered by the lettuce. However, I don't have to worry about this too much right now, because this is year one in all of my beds. I'll get into the more subtle nuances of planning and crop rotation next winter. Until then, you can check out Eliot's books for more information (yes, we're on a first name basis now.)