Do you mean an operator? If so I make a massive judgment based on the software they use a lot of the time. I also care about their overall reputation, are they AGD approved etc. It also helps if the person that approaches me to work with them is knowledgeable and professional
1. How do they treat their players? I only advertise places I feel reasonably confident will be fair to their customers.
2. What player welcome bonuses to they offer? Too many casinos these days have crap sign-up-bonuses = too hard to get players to sign-up.
3. Do they have NCO or any other affiliate unfriendly terms? It's just not worth taking the risk with bad terms - why should we when there are plenty of decent programs to chose from.
Oh yes and if you are new in this game NCO which KasinoKing is mentioning. That is negative carrier over. You wanna avoid that as it can really put you down getting big loss in your first months after having players deposit. But as always just you your negotiating skills before starting to promote a program. Say you will only be dealing with them if they remove that terms also minimum players. I have done that with couple brands. Though it can be really hard getting that done if your brand new and only just started with a new website
So to summarize:
The reputation
The software
The human aspect:
With the players (respect + professionalism)
With the affiliates (respect + professionalism)