Fun
I love how the game evolves from a simple shooter into powerful instruments of war. The upgrades are a bit folly, aside from the in-game ones, though... The game was also a bit easy. Maybe some kind of medal like "Beat the game without losing more than 5 times" would be cool.
And here's the best thing you can do:
Line up guns side by side until you have 4 on each side... Start them off as level 1s, you don't need to build up each one slowly before adding another. Just direct the kill zone wherever, and the enemies drop like stones.