Rarity philosophy:
Commons are the cards the players will be introduced with. They are most of the cards of each element.
Uncommons are cards that newer players will want to have because they 'look better' or particulary cool or offer something that commons don't. For example Dragons, Lava Golem, Phoenix, all Alchemy cards, FFQ, Golden Hourglass, Dim Shield, EQ (I may make a full list of the ones I think should be, when I get a computer in 1-2 days)
Rares are cards that can be a good addition in a deck without being a core card, i.e. Miracle and artic squid as they already are.
Ultra rares are... that. Harder to get. Shards are good URs to start.
Legendaries are just ultra cool not farmable cards. You get them in Tournaments and Oracle (or other new ways in a future)
Cards price philosophy:
Each rarity of cards has a different selling price range. So (for example) common cards are sold between 10 and 25 electrum each and uncommons between 30 and 60. This way, players will have in their mind 'on average, uncommons can be sold 2.5 times the comons are'. Of course this numbers I just said can be altered, but you get the point. Same with the rares and URs.
Booster pack philosophy:
We want the players to get a lot of commons when just starting, and when they feel they have something decent they will go for uncommons and rares. So I think the pack method used in BattleCry (search it in Kong) is optimal (for everything I say about the packs, it will be for each of your 3 element-sorted idea):
Bronze pack: lots of commons. 12 is a good number.Cost: about 4 won games againt AI3.
Silver pack: some commons and a couple of uncommons. 5 and 3 sounds good to start (atention: balance the quantity with the price philosophy!) Price: pretty much the same as the bronze packs. Same price, less cards but hey you start getting uncommons!
Gold pack: 6 uncommons and 1 rare. Price: balance the average with the cost philosophy.
Platinum pack: 3 uncommoms, 3 rares, 20% chance one of them being ultra rare. Price: expensive, to analize later
@Fippe:
I think the Time Bomb buff when upgrading should be on it's cost, not in the damage.
And yes, the Poison buff at targetting creatures is the one I just suggested in GS&F, and all the feedback I got was 'the AI is too dumb to use it well'.
I wrote all this in my phone, I feel awesome x)