Yes, it's all in the guidelines.
But it basically always boils down to two reasons -- either not enough information that is unique (it's amazing how many people have trouble with the u-word, but the i-word causes problems sometimes) or having content that is really better associated with another site (that is, doorway pages, vanity domains, affiliate sites, businesses that operate different sites under different aliases, and the like.)
Beyond that, it's really unnecessary to say. The webmaster knows where his content came from, and also knows who is the commercial beneficiary of the advertising on the site -- knows more than we do, and often knows more than we can know. You had best ask him those kind of questions.
We look at it the other way 'round. The submittal didn't help our editor find good content. No hard feelings, but if it's not helping us, it's time to close the conversation.
