I think most important two factors are:

1- The purpose of the ICO, many are similar to other ones with one or two minimal differences just to say "Hey look at us! money money money!" That would be your own Judgement in the End.

2- The Team, Team, Team. Past experiences, positions and projects done by team members, you do not want the Backstreet Boys starting up an ICO.

This is just my opinion, many other factors also follow, but these two are the "NO" makers for me, if they aren't right, it is not worth looking into, if they are good, I might consider reading the rest of the Whitepaper :)