You can be a positive fan and still be angry. Christ, I would say that if you weren't mad during last season you aren't that big of a fan to begin with. Should we be flipping and burning cars in the lots when the game doesn't go the Bills' way (well, I'd say yes, but that's just me), probably not.
But, if the product blows, and you're paying to see the product, you should be able to say "This product blows!"...and then run the sons-of-b!tches responsible out of town on a rail!