Stability

It's always the games that are really interesting when it crashes, isn't it? You can fight 12 Nissas in a row with decks full of Pharika's Disciples and Caustic bloody Caterpillars and you beat them in 8 rounds and the game is stable as anything. But when you come up against a Jace who drops 2 Ringwwarden Owls in the first couple of turns and you're having to fight for your life and make real genuine choices in order to survive, *that's* when it crashes.

So long as you're grinding away at the game bored to tears it's pretty stable. But if the game gets interesting at all, it's probably going long, and that means you'll get a stack overflow or memory leak somewhere...