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First off, every game needs pause and mute as keyboard shortcuts.
The weapon-switching gameplay is actually loads of fun when it works, but for the majority of the game it seems a bit pointless. Maybe if you tanked out the enemies a bit and increased their spawn rate while at the same time reducing the recharge on the weapons, it could be a more exciting game. Maybe even go all out and make some enemies that are only weak to specific weapons (like some chilli con carne that's really hard to take out with everything but frost) and a difficulty selection would be brilliant.
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BORED. Dude spice up the gameplay (sorry bad chilli pun) add something to let the player know they are progressing... anything! like a bacon sandwich maybe? hmm i think im getting sidetracked...