There's so much in Shadow Warrior that was revolutionary; ideas and gameplay ploys that have now become part of the firmament in terms of how FPS games work. There were drivable vehicles with guns on the front, and the earliest usable gun emplacements that my fragile mind can recall. Then there's melee sword combat that's actually vaguely viable in-game, alongside differing death animations for the poor zombie ninjas you use it on. Heat-seeking missiles, gas grenades that would pollute the air around them, sticky bombs that attached themselves in the fashion of Halo's plasma grenades, vehicle-based multiplayer... The list goes on.
All this was aided and abetted by what was now 3D Realms' utter mastery of the Build engine. Cars would crash in a wall of flame, vast underground drills would be turned on and dig down into the earth to open up a passageway, tube trains would appear out of nowhere and have you jump out of the way - Shadow Warrior's simple scripted sequences were furiously imaginative and by and large of a quality previously unseen. For example, the game's non-shareware opening pulls you downstream on a boat, before pulling different circular chunks of water texture down one after the other to create the impression of a whirlpool while your ride spins in circles. Not all levels show this amount of flair - there's a hell of a lot more filler in Shadow Warrior than there is Duke Nukem 3D - but overall what 3D Realms' developers lacked in cultural awareness they certainly had in unexpected and original level design.
What I find fascinating though, and apologies for my nerdly fervour on this one, is that contemporary reviewers really thought the game somewhat backward. The advent of Quake and its fantastic grimy polygons had arrived, and sprites were suddenly hackneyed and quaint - only a couple of steps away from a game of hoop and ball and a ride on a Penny Farthing. The game's innovations were pretty much invisible to the chin-strokers of the day when put alongside the stunning id tech, during a period when, I would argue, the FPS stuttered while everyone came to terms with the 3D revolution. It was only really when games like SiN and Half-Life came about that the groundwork laid by games like Shadow Warrior could truly be built upon.
It did a hell of a lot of cool stuff very early on in the life of the hallowed FPS. Comments below this article will no doubt suggest other games did clever and revolutionary things first (Terminator Future Shock certainly had drivable vehicles a few years beforehand, for example) but my argument is that Shadow Warrior put it all in one place, and nigh-on perfectly. As a result, it's virtually a museum piece - as you play, you can see direct lines between the shooters of yesterday and the scripted cinematic wonders of today. And if it weren't for the boobs, gay rabbits and questionable attitude, then right now it would probably be on show in one, rather than buried under the house.