Of the top of my head
1) In depth stats, can’t stress that enough. I wan’t to see refering pages, players country, games played, basiclly as much info as possible.
2) Ability to ‘deep link’ to certain pages. i.e. send the visitor to a ‘promotions page’ or ‘games pages’ rather than just the homepage.
3) Fast payouts, preferably before the 10th every month (bit of a no brainer)
4) Reliable tracking (again a no brainer)
5) Lots of creatives, I wan’t banners of all shapes and sizes, updated frequently for new events/holidays etc.
6) Emails from affiliate managers. Although I get emails from most programs it’s still nice to hear from time to time what’s going on, latest promotions etc and an email from the affiliate manager would always be nice once in a while (mainly to kick me in to gear 
)
7) Incentives. Although I’m not usually one to fall for extra incentives they are ofcourse nice, although I would prefer a permanantly nice revshare/cpa than the occassional monthly promo.
Brand name importance, This is a big one with me, the brands I am promoting need to be well recognised in order for me to sell them to my players.
That’s all I can think of for now….