The most important thing for me is the product itself, I buy the product and if I'm happy with it then I'll promote it. Another thing I check is what else will the person who buys the product need after purchasing the product.
That will help me come up with compelling bonuses which will increase conversions. But most of the time I want a product that is complete.
For instance, the product I'm promoting at the moment teaches list building. The buyer gets training, free autoresponder, free software for building squeeze pages, a free high quality giveaway/freebie product and everyone who bought the product gets 100% commissions when they promote the product they purchased. While other affiliates get 50% commissions.
The training has some great traffic generation tips. As you can see, the product is complete and the buyer will have all the tools they need to make money with email marketing.
These days, everyone is selling information and that's no longer enough. People also need the tools so that's what I mean by promoting a complete product. I promote a vendor who brings something fresh to the marketplace, not just information.
There's a lot of rehashed stuff online, I look for innovators, people with fresh ideas. So what makes me sign up as an affiliate is fresh ideas.