I played Nightfall recently, and couldn't finish the game because my brain hurt. This is coming from someone who is very well versed in the intricacies of the Magic: the Gathering rules. After it was mostly cleaned up I took all the different cards and found something resembling a pattern in how the various attributes of the cards were distributed so maybe I'll have a better chance if I play it again, but at first glance there are just so many relevant aspects of each card that it's hard to come to grips with exactly what you really want to do. Basically, the problem comes down to the system of how you play cards from your hand: each card is a specific color that is only indicated in the top left corner, and has 2 different colors next to it that it indicates what color(s) of cards are allowed to be played next. Each card also has a "kicker" color, wherein if you play it after a card of the given color, you get an extra (or at least usually slightly better) effect. That's 4 colors on each card, and they're all different, and there's 6 colors in the game. One player chains together as many cards as he wants, then each other player does in turn order, stopping when the last player passes. Then the chain resolves Last-in-first-out, meaning that what you wanted to do with your cards when you played them initially might become totally irrelevant.
Now, it wouldn't be all that bad, except you have private supplies of cards that you picked, presumably because you want to base a strategy around them, but when you picked them you didn't know what was going to be publicly available that might allow you to chain into them. So in considering what cards to buy, you have to figure out whether it has an effect you want, can be chained into or out of by other cards in your deck, and how your opponents are going to mess with your plans. My suggestion if you end up playing the game is to not think very hard and just buy what you think is cool. There's way too much thinking involved for very little reward otherwise.
It's also somewhat political in multi-player, as you choose which player(s) to attack with your minions. In a 4-player game you probably can't win if the other 3 don't want you to, something I find a total turn-off in a game.