Each character starts with two suits of cards (one red, one black for clarity). Presumably the different colours or suits mean something (lets say power and finesse). Face cards (Jack, Queen, King) are the possible negative effects. Character creation determines which cards are eliminated at this stage, so someone with a lot of power but not much finesse might have lots of high Black cards and low Red cards. This becomes your base magic deck.
The "clever" bit is what happens next. Preparations you take when casting eliminate cards from your base deck, so a fast cast has a much higher risk of causing problems while aiming, for instance, might eliminate some red cards. You might be flexible choosing which cards to eliminate or you could make it prescriptive, which is my preferred choice (curiously). The latter would mean each character would have a crib sheet of activities they need to make time for to pull off the perfect spell. While all this may seem time-consuming, what it does is mirror the delays in casting magic to the delays the player takes and reviews have indicated that this is effective in other games.
Finally, my vague thinking for resolution is that you shuffle up then flip the top two cards. If both are number cards, choose the best result. If both are face cards, both bad things happen. If it's one of each, the number card triggers but there is an element of the bad in there. For most spells, you can determine the finesse and power needed - not all magic is about maximum power and if the world assumes magical power comes easy you might be constantly aiming to restrain the power not increase it. (Half) the difference between the revealed card and the power or finesse needed indicates how well you did (the value not revealed is assumed to fit - after all, you ARE concentrating), but any numbered results is a success of sorts.
While I don't have a world to go with the system, I do have one more thought: this would also work well for super powers, which also have the power/control element. This is probably the area I would look at first. Of course, I would need a base mechanic to go with the superpower mechanic (unless I go the Nobilis route and assume prowess at base human activities).