I get asked this question occasionally via e-mail, so I thought I would give you my long winded answer :) Well first of all, I have to be able to program the card. Fireball and Mind Twist are both iconic cards, but currently I can’t implement X spells or abilities. Many card choices are eliminated like Eternal Dragon (5/5, pay 3WW: move this creature from your graveyard to your hand), because they are too complicated.
Second, the card has to be powerful or fun. No one really wants to play with Eager Cadet. He is a feisty 1/1 but who wants that when you can use Isamaru, Hound of Konda or Savannah Lions who both attack for 2. Practically no one has ever played with a real card board Ancestral Recall or Juzam Djinn and my program lets you experience that kind of rush.
MTG Forge also has all the Moxes which are insanely powerful. Mudbutton Torchrunner (1/1, deals 3 damage to creature or player when destroyed) strikes me as funny and Hex (4BB, destroy six creatures) is just awesome if you can pull it off. A few times the computer really devastated me with Hex. Relentless Rats is the ultimate fun card and now you can build that deck that you have been dreaming of since it was printed way back in Fifth Dawn.
I also try to program cards that generate lots of interest like the planeswalkers and cash rares like Thoughtseize and Damnation. I added Flametongue Kavu because the magazine Inquest (when it was around) talked so much about 187 creatures. (187 is the police term for murder, so a 187 creature kills another creature.) Flametongue is just an insane card and those are the kind of cards that catch my attention. When Kiki-Jiki, Mirror Breaker first came out there were a ton of questions about him. I love using Kiki-Jiki and I’m glad that MTG Forge has that card.
Finally, I try to pick cards that are good for sealed and draft. Limited has always interested me and I have always thought of MTG Forge as a limited simulator. Constructed (all of MTG Forge’s cards) can get old but the limited portion of the game was the most interesting part to me. Limited games seem to offer more variety and surprise than constructed since you don’t know what cards you are playing against. The only constructed games I play use the “Generate Deck” that I so enjoy.
I played Shandalar a lot when I first downloaded it but I got bored of the constructed games. Shandalar’s sealed deck option was very fun even though the sets weren’t designed for sealed (I always seemed to have more red cards that any other color.) Some cards, like walls, really annoyed me and I wanted to remove them.
MTG Forge lets you add or remove cards by editing the files common.txt, uncommon.txt or rare.txt I am very happy that MTG Forge lets you mulligan unlike Shandalar. Sometimes a mulligan can make all the difference between winning and losing.