Recently I've become addicted to an online card game called Tyrant. I've been playing it on Kongregate, though I assume all the cool kids are playing it on Facebook instead. In any case, it's a collectible card game (CCG) similar to Magic: the Gathering (MTG), but streamlined for more casual play.
I'm interested by some of the design decisions they've made with Tyrant. First of all, they've removed a lot of the choices a player of MTG might have to make, leaving the player with only the decision of which card to play from his or her hand at the start of the turn. All other game rules are deterministic or randomized. So the strategy is simplified, which I expect makes it easier to write an algorithm to allow for computer-controlled opponents. The algorithm just needs to pick one card out of three to play, on every turn.
Also, they've made some cards measurably more powerful than others. In theory, this is balanced sometimes by increasing the activation time of the more powerful cards (in other words, it takes longer for these cards to become active after they're played). But even when comparing cards with the same activation time, some cards are definitely more powerful than others, which leads to the player eventually retiring some cards as "obsolete" in favor of newer, stronger cards.
I don't know how prevalent this is in MTG, but I would think that this would be undesirable from the point of view of a traditional CCG that's used for competition. After all, the point of a CCG is that the strategy is in deck-building and intelligent play. All cards are supposed to be good in certain circumstances, and some combinations of cards can create a synergy if played at the correct time. So, ideally, each card when taken in isolation should be just as powerful as any other, so that just having card X does not automatically give you an advantage over players who don't have it. Having intrinsically overpowered cards makes the game less about strategy and more about acquiring cards.
This aspect of the game actually works more or less fine in Tyrant, since it's not a tournament game. The idea is to complete single-player missions in order to acquire more powerful cards, and thus improve your deck so that you can beat harder missions. There is a player-vs-player aspect, but in situations where one player has collected more of the advanced cards, that player will have a strong advantage.
You can pay real money to buy cards in the game, but you can also just get new cards by playing the game and beating missions, so you can accomplish quite a lot for free. As a result, I've been on this game every day since I found it, opening booster packs and assembling decks and such. I do have a weak spot for this kind of game; it's a good thing I never really got into Magic, or I'd probably have a much lighter bank account by now.