Very few FPS games have a distinctive sense of humor. Duke 3D set the (low?) bar for cheesy one-liners and gross-out, whereas ROTT is all about a goofy sense of fun along with the gibs. The first FPS I can remember with dual weapons, and some absolutely insane special items (god mode and dog mode especially). The attitude even seeps into the main menu, with various death sounds playing on exit. Good, solid fun with a simple level editor, too.