I want you to stop reading this for a second and go play "Asteroid's Revenge." I'll wait.
Welcome back. Now, while the game did not have a lot of spit and polish (missing a life meter, shots originate from the center of the ships instead of the nose, horrid UI), it was still fun, even though it was just the original game of "Asteroids" flipped inside out.
This is the kind of thing that can spawn a successful game...taking another game and reversing the roles of the protagonists. However, it doesn't often happen in our industry. Why?
We try to pigeonhole our games into these set categories. (FPS, run-and-gun, stop-and-pop, RTS, RPS, RPG, MMO, match-3, card battle, top-down shooter, etc.) Mashups and reverses rarely fit into such narrow categories, but they can be so fun.
Even simple non-game time-wasters like "Kitten Cannon," with the infringetastic Daft Punk music, are difficult to categorize. (Yes, it isn't a real game...but it's damn fun.)
So, the industry insists on pigeonholing games into these set categories to be marketed, but they're fun? How do we get the games out?
We have at least three choices. We can go full-bore with a full development budget and hope that when we ship, the public happens to be in the mood for feline artillery or rock-beats-ship. We can go cheap and create the fun game with the gameplay only but no assets to speak of, and not make any money because people don't like paying money for something that looks like crap, but then watch a competitor slap a fresh coat of paint on our idea and have it take off.
Think outside the box for a second...what do you think the third option could be?
(And yes, while I did not take the cat picture, I captioned it. It seemed appropriate. You can get a metric litterbox-ton of cat pictures, including the source for the one I used, here.)