I don't think I like the mechanic.
It seems you can't get a full set of cards by playing a game. I got 3 out of 5 cards by playing Stay Alive yesterday. I didn't have to do anything, and didn't know I got them until I stopped playing, I just had to have launched the game.
However its not possible for me to get the remaining 2 cards, no matter how much more I play the game. There are 3 (random) cards given out by playing. The 2 you miss can only be gained by trading with someone else who did the same - but doesn't want them. I now have to trade or buy. However since I can't earn more cards, what do I have to trade with? Thats right. Cash
The FAQ sugests you need to play other games that have cards, to collect items to trade. You will never have spares or duplicates. You will just have cards that you are willing to give up from 'lesser' games so you can finish your set for your primary game.
It is totally relient on those people who decide they never want or care about the cards. It is not a zero sum system. It is not possible for everyone who participates in each game to complete the sets.
Then there are the rewards from all this. By gaining levels in Steam, one of the benfits is an increase in the number of Friends you can have in Steam. As far as I am aware, this used to be unlimited. So now they are putting in a limit, so you can get the reward of increasing it? Let me keep my present no restictions thanks!