Most important is a good site that is well targeted to the merchant's products.
Some won't approve you for auto loans if you apply with a golf site, for example.
Some merchants screen out affiliates based on country.
Some screen out based on your Alexa ranking,
PR and/or # of green bars, if you are a CJ affiliate.
Some programs are lean a mean and only accept the cream of the crop.
Some programs are wide open, auto-approve everyone and don't screen at all.