One of the hardest parts of a "seed investor" is having actual successful experience in an early stage company (See PG, Trevor, and RTM in ViaWeb-Yahoo Store), and being able to use that experience when mentoring a young company.

Having talked with all kinds of investors it's amazing how much more you can respect and understand the advice of someone who's been in your place before - and how much more relevant they can make it through their own experiences.