Do they value and help their affiliates? Some affiliate programs do an exceptional job of communicating with their affiliates, notifying them of upcoming sales, offering marketing advice or tools, offering contents and prizes during promotions and more. These types of affiliate programs are a pleasure to be a part of. Ultimate Bundles is an excellent example.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.