I'm not going to get into the specifics... In short if you produce a decent looking product that doesn't infringe on any licenses there shouldn't be an issue. We will not be reviewing based on content, but more on presentation and functionality. If a book is writen by someone who obviously never read the d20 license we'll certainly not post it, though we do that already as it's a concrete thing not subjective.
Honestly I don't see many products being turned away, but obvious amature crap will not be getting past the doors this year.
Sage might have more input then I do as he'd be one of the people on the review team - at least our front line.
James
P.S. BTW, Our new site revamping and our search engine optimizations and the ENWorld sale should have everyone rejoicing in a great month. I think this is going to be a record month for RPGNow and a great way to start the new year.