Around 2% of the American population has some kind of vision disability (i.e., are blind or have significant difficulty seeing, even with glasses)
Around 50% of the population has some kind of clinically significant refractive error (a visual impairment which may be corrected with glasses if mild enough)
Around 8% of males and 0.5% of females have some form of color vision deficiency
Around 2% of adults have a hearing disability
Over 4% have a cognitive disability (difficulty remembering, concentrating, or making decisions)
Over 5% of adults between the ages of 18-64 have some sort of ambulatory disability; the number is nearly 23% for those over 65
Everything should be accessible via the keyboard alone (no mouse/cursor)--ideally, even if the user can only access a single switch
You can move focus around the page using TAB (forward through document) Shift-TAB (backward in document) and arrow keys (inside an element) [Note: in a Mac, you need to add the Option key to TAB, or go to System Preferences > Keyboards > Shortcuts and click All Controls]