Take Radiant Silvergun, for instance: originally an arcade game and then a relatively limited Japanese Saturn release in 1998, the game became a favorite among the ultra-hardcore, import-happy crowd... assuming they could get the game.
Radiant Silvergun also defies neat categorisation. Aesthetically, it's an orthodox shoot-'em-up. But it evolves the genre in fascinating ways that, perhaps due to the dearth of companies still working in the area, have never been borrowed or stolen.
The other major aspect of Radiant Silvergun is the color-coded combo system that rewards you with bonus experience points for consecutively shooting down enemies of the same ilk.
For the uninitiated, Radiant Silvergun is absolutely daunting the first time you start things up. The right side of the screen displays all of the weapons you currently have access to, as well as the buttons that they're mapped to. Your eyes stare at the screen, trying to comprehend how you're going to manage seven separate guns at once.