Tough (but very good) question!
Off the top of my head (which means I might change my mind later...

)
1. Easy to access & understand game logs with
full at-a-glance details - like Crypto's!
2. Ability to change the 'game window' to any size you want without screwing up your computer's display settings - like WagerWorks.
3. Players able to chose to only download the games they want - like all the main softwares except MG & Playtech (I think...?)
4. Clear banking screen which shows exactly how much you are allowed to withdraw, and software which processes & makes the payment within 24 hours! (No reverse withdrawal period).
5. Software which prevents players from playing in any style which the casino deems abusive, so there is NEVER a case for confiscating winnings based on 'irregular gameplay' or any similar BS excuse.
Sorry, 5 is not enough!
6. Good choice of completely unique slots - with near infinitely adjustable betting levels - like most MG slots.
7. Software which starts up very quickly without doing loads of very suspicious & apparently unnecessary 'upgrades' (are you listening MG & RTG?)
In conclusion:
If only they would slightly upgrade their older slots (to spin a little faster and be less jerky) and introduce almost infinite slot betting levels like MG have, then CryptoLogic would be just about my perfect software.
KK