A quality affiliate will vary depending on the niche I imagine. I think with poker, 3 pros beats 100 feeders am I right? So you could probably be an extremely productive affiliate without huge numbers.
I think in general though, beyond the obvious ability to generate converting players a quality affiliate is one that might benefit the brand image in addition to generating players. Towards this end, a quality affiliate should have credibility with the target market and present the brand in a positive light where possible. That takes trust. Which is why quality affiliate managers attract and keep quality affiliates.