The difference between A L I E N and Avp2.
Not to say that AJL is without its problems. The humans have WAY too many grenades and the things you can't spam in AJL are replaced with OTHER things you CAN spam. But it still plays much closer to the movies and is much fairer to all three races in general, but it isn't perfectly balanced by any means. The methods of weapon firing are also a lot closer to say, Counter Strike, than the vanilla game.