- Don't make things work in a different way than your users are expecting (i.e. breaking the "back" button when using Ajax in web forms
- Follow the K.I.S.S principal
Really, any rules someone posts will be a variation on the theme:
Don't Make Your Users Think
"Don't Make Me Think" has already been posted, see also
Design of Everyday Things and Designing with Web Standards which are also great for light usability reading.
