What I found so impressive about the original Doom when I first played it, was the level of immersion from the engine and photo-realistic weapon sprites, as well as the atmosphere created by the horror, and of course the stellar level design and satisfying gunplay. I've since come around to liking the new Doom's power-fantasy Slayer/God version of the Doom Guy because he's also badass af in his own right, but it's a very different take on the series' protagonist. That you actually survived the forces of hell no mortal should have to face is what makes you an impressive action hero. You are all alone, doomed, and surrounded on all sides by demons out to get you. It was very much keeping in theme with the sort of themes in darker metal and grunge of the early 90s. Just some non-descript marine, and you were the one Doomed.

Like Tetzlaff said, the original Doom soldier was just an ordinary human. (Though I'm aware it got a lot of flak even at the time). I think a lot of Doom fans now have a different perspective than perhaps when Doom 3 came out.

Doom 3 realistic weapons