I had a fun time selecting which cards to program. My focus were cards that would be fun for limited, because they would also be fun for constructed. My initial goal was 150 cards, 30 for each color. The card rarity was like 20 common, 10 uncommon, 5 rares. I knew that common cards were the ones that were used the most in limited, since they are the most available.
For the rares I wanted game breaking creatures and spells like Wrath of God, Serra Angel, and Tromp the Domains. I thought of uncommon cards as usually being better than the average common card. An awesome card that I downgraded to common was Infernal Kirin. Clearly Infernal Kirin is more powerful than any common known to man. I also classified cards as uncommon if they were high costed or if you would only want one of them in your deck. You would only want at most one Puppeteer in your deck and while Craw Wurm is sometimes a big threat he is fragile, so he is uncommon also.