A program should satisfy both advertisers and members.
For Advertisers:
Web site designed professionally, runs on a well known script (like Cash Crusader),
Hosted on fast servers after all advertisers pay members to see their adds but poor or stolen scripts and slow to load sites means timers for adds finish and members are credited most of the time before the advertised sites load.
Acceptable support services for advertisers
Easy to navigate order page
Accept different payment options (example programs that accept payment by e-gold only are discarded)
Experienced webmasters known for their honesty
Others factors to consider: Balanced pricing and rewarding, level of referrals, minimum payout, average reward of a click, websites updates.
For Members:
Honesty with reading members is a must, paying on time, regular updates.
Fast servers, no frustration while waiting for your clicks to clear
Moderate minimum payouts
High pay per click (no tiny fractions of a cent)
Must have paypal as a form of payment, which is a secure payment system that signals confidence.
Minimum payout is reachable without referrals, of course within a reasonable time
Important warning!!!
Making money on these sites is real and 100% legal; but keep in mind that would not mean you are going to be rich overnight.