If you plan to market the game, I'd strongly suggest network play. Co-op is something that is either not offered or poorly offered(same thing as single, but doesn't offer a good 2+ player experience).

It's always good to criticise yourself in development(NOT too harshly! ). But pick points from the outside point of view that could be a feature that *sucks* if you were some random dude plaything this game you downloaded off of tucows.com... And if you can improve it so tht you really can't pick it appart, you've improved your game and others will like it more. You probably will too.