Doom II's popularity foreshadowed this underappreciated classic back when it was released. No ortogonal walls!! One of the biggest criticisms of the game. What Rott lacked in variety of angles made up with some cool new features not seen before and a sheer over the top style and tone. Doom's environments were mostly static, the only things that the player could interact with were the exploding barrels. ROTT had those too, as well as exploding chairs, pillars and glass windows. Each level came to life with the use of moving traps, rotating blades, moving walls which could melt the player if they got to him/her, floor and ceiling traps, etc. All of the enemies you fight are human, or human-like, unlike the funny looking enemies from games like doom and duke3d, which might have been meant to counter the excessive nature of the game with some recognizable characters.
Where really ROTT shines is, nonetheless, it's amazingly fun multiplayer feature. Some would say that the game was developed as a multiplayer game only, and then a quick story mode was added in the final minutes of its development. It's comm-bat mode, as it was known, featured some of the most powerful arsenal of rocket-based weapons not seen in any modern fps game of today. The firebombs, which bring down death from above, and specially the flamewall, which creates a deadly wall of destruction until it reaches the end of the level make up for hours of sheer online gibbing goodness.
Oh, and it's really cheap too.