I get asked this question by e-mail and I'm sure many people think the same thing. I have two main criteria for adding new cards. One, can MTG Forge support the card and two, is the card fun and/or powerful?
MTG Forge can only implement straightforward Magic cards and until very recently didn't support creature enchantments or cards that produce more than one mana like Dark Ritual. MTG Forge has a number of cards that are complicated like planeswalkers, Thieving Magpie, and Hypnotic Specter but MTG Forge will probably never support a complicated card like Warp World, which is very powerful but also has some insane edge cases.
I know what kind of cards that MTG Forge can and cannot support but that information is very hard to explain to someone else because there are so many exceptions. MTG Forge can support Hex but cards are usually restricted to only one target. Time Walk cannot be supported because the phase code is very fragile.
I also try to only add cards that are fun or powerful. Maybe someone gets a kick using Eager Cadet but that card is very pathetic and weak. Royal Assassin is a simple card to program but it is extraordinarily powerful. Likewise the code for Wrath of God is very short but it is one of the most influential cards in Magic.
1. MTG Forge can only implement a small subset of all Magic cards.
2. The card needs to be fun and/or powerful.
On a side note, I was sad to see that Wrath of God wasn't included in the new Magic 2010 core set.
MTG Forge now has the all powerful Morphling!!!