Chris DeVore has published a post on the unwritten rules that most investors have when deciding whether or not to nix a possible investment in a startup. According to DeVore and other investors that he references, there are a few basic and key decisions at the early life of a startup that can kill a potential investment. His advice is to always make sure you learn about the unwritten rules and do the best thing possible to avoid breaking them. Or to have a very good reason for breaking them.
[Source: Crash Dev]