1. Updated content. Be it a fanfiction site, an informative site, tutorial site or whatever you absolutely must update on a regular basis. I realise that if your page revolves around your fanfiction or fanart it may be hard if you hit an inspirational block, but you could simply mention that you may take a while updating due to that block in your updates page or whatever, right?
2. Interesting content. Speaks for itself really. If all you've got is some bland, boring data and some bad quality images you don't stand a chance it becoming an affiliate.
3. Pokemon content. Although I am a DBZ lover (with a DBZ page, too) and enjoy plenty of other things such as Final Fantasy and Pirates of the Carribean I will not accept non-Pokemon sites for affiliates. This is simply because this site is Pokemon-revolving so it wouldn't really make much sense. I will add you to the link menu if your site is good, though. ^^
4. A nice layout. I know, I know, that sounds very shallow but I'll only accept sites that look good too. By look good I mean tasteful colouring and interesting font. If you're no good with HTML but have still managed to give your site a nice look using just colours then that's fine too but if it's boring Times New Roman in boring black and white without any pictures or anything you will not be accepted. Simple!