When choosing the cards for MTG Forge for limited games, finding the right cards to fill up the rare and uncommon spots was easy, but the commons gave me trouble. As I programmed new cards past the 150 mark, currently I’m at 365, I would add them usually as a common first, so I could see how they played. All of the killer commons like Sakura-Tribe Elder and Man-o'-War were already included. Sometimes I would add a card as common but it would be too powerful so I would move it up to uncommon. Most of the time I would just delete the card from the common file because I didn’t enjoy playing with it.
It is hard to say why some cards are fun yet others aren’t. Kodama's Reach is a great common and is useful in a ton of situations. Dryad Arbor which is a new card from Future Sight that is both a Land and a Creature, not super. I played with Dryad Arbor and I couldn’t find a use for it in limited games. It counts as a land so you can’t tap it for mana because it as summoning sickness which sucks. Dryad Arbor is a great “way out there card” that sucks in limited.
I didn't include Delirium Skeins as a common because I was tired of the computer playing it thus making me discard 3 cards, yes 3 cards. It seemed to make the match take too long, since you were just drawing off the top of your deck. Oddly enough if the computer actually played Delirium Skeins, it would usually win. I can add almost any card I want as uncommon or rare because they don’t effect limited games as much. Finding fun commons is a lot harder that finding nice uncommons or cool rares.