Wow, I have a black card for 1 B that gives -1/-1 to all creatures. It'd be dead before turn 4.

Of course, that's assuming I'm playing a black deck... I think Green's the only color which has trouble with stuff like that though, since red is basically all damage, black is kill everything several times, white has nukes, and blue would kill it before it even got into play. But really, who plays mono green?