Great thread indeed!
Basically, the 3 most important factors considering reccuring programs is:
1) conversion - ok, thats the basic. 1:600 or lower or goodbye
2) retention - thats another very important thing, some people only care about conversions and then they end up making $1.95 per signup.
3) cost to get a signup - if the sponsor program offers free content, or sponsor hosted galleries or bannerless freehosting, that means the promoter spends less to make a signup, and thats good stuff!
The rest is additional stuff
What do you people think? This is a very interesting thread, post your comments and opinions!
Take care,
Bruno