2) There is only one more site that delivers the same functionality and he is doing it poorly. If you really believe that your solution is better, go a head, build your own application. Don't forget, however, to try and understand why this site is doing it this way. it is possible that there is a reason. Be careful not to find it after spending too much money and effort.
3) There are only two other sites that deliver the same functionality and they are making tons of money. Sites like that will be delighted to share the market with you and your application. It is true, sometime they will rather buy you and get rid of the hassle, but most times it would be very easy for them to have one of their own developers develop a similar product and use their market power to kick you out.
4) If you can describe you site without using other successful web 2.0 sites. If you find yourself describing your site as "Something like digg for..." or "it will work just like delicious but ...", it is time to find another idea.
5) If there are no more then 2 web 2.0 buzzword in your site description (not to mention, in your site title). Let's see you not using: ajax, social bookmarking, user-generated, shared video, mash up.
6) If someone who never read techcrunch can understand what your site do. remember, techcrunch reader are your competitors not your audience! if you can explain what you site do to your non techi friend, you should be OK.
7) the combination of 5-6, if your site is meant for delivering bloggers feeds using ajax user interface and google api Mash up designed for sharing rich content in social network enviroment" you are in trouble. Even Michael Arrington will need a minute to understand this.
8) If you don't know who is Michael Arrington. Or maybe ...
