My answer to How can I know if my web startup idea is worth launching?

I like your points (3), (4) and (6). I thought (1) and (2) were true, but in a sort of vacuous sense... that is, you can ask yourself those questions, but - for most people - the answer will simply always be "yes" because people tend to fall in love with their own ideas. It's hard to be objective when asking yourself questions about yourself. Maybe some rare people can truly pull it off, but I like the emphasis on (a), external feedback, and (b) "does it scratch an itch?"

Short of doing some formal market research, or something like @sgblank's "Customer Development" stuff, I think those points are the best place to start.