Personally a portfolio site should be three things, firstly it should demonstrate the quality of your work over a varying range of applications (types of site) and time, secondly be clean and easy to use and set out or presented in a professional way.
Lastly add sites or projects that interest you, no end of web designers (and programmers) simply put their big clients or "pretty" work on display and really lack anything to say when questioned, if there is work that makes people want to ask questions (description etc) then they can quickly gauge your enthusiasm, commitment, skill and knowledge when contacting you for more details or further information.