In game design, conditions. Not because they are bad in of themselves, but it can be hard enough to remember other restrictions as it is. It's adding unnecessary complexity to an already complex (but very rich and good) game.
In practice? Poor winners and poor losers. Both of whom can sap the fun right away. Actually played a game at lunch today and seriously thought about not bringing ships to work again for lunch games with how one guy acted. Also, it gets old fast to see people complain about how things are "broken". I don't think that any fix has broken xwing... The only time a list was truly overpowered has been handled. Even while I may not like conditions, I don't think they're breaking the game (yet, if it goes too far down that path it could get ugly).