Now and then I wonder why Doom was such a perfect game, mostly I think it is nostalgia besides revolutionary technique at that time. I've also read the book "Masters of Doom" and would recommend anyone who wants to get inspired on creating something from almost nothing by some sort of following your heart and using your (smart) brains. It makes me happy to read the book.
The main ingredients which imho make Doom classic:
- creepy/spacey/metal Adlib music, thanks to Bobby Prince (Adlib was good for that)
- simple, but effective spooky samples playing at random locations
- for the time good textures (need some good graphics artists) to give it a dark feeling
- kiss play experience (see how it probably would be done nowadays: http://www.youtube.com/watch?v=C4yIxUOWrtw )
- VGA-graphics, I keep saying this to people: VGA is pixely, but somehow I don't see that when playing Doom, I compare it with cartoon drawings, you don't focus on the details, but how it looks globally and use a bit of fantasy to bring it to life - nowadays graphics are so detailed that I somehow get distracted by too much detail, it's becoming too realistic almost and doesn't look like a 'game' anymore
- deathmatch or coop via modem, calling your friend to play a match via the telephone line, warning your parents that they are not allowed to pick up the phone, playing not too long because you had to pay per minute (in The Netherlands at least)
- everything packed in a few diskettes (3 or 4?) was at that time almost magic that you could have such a 3d world fitting on it
There are many more reasons why for me Doom will always have a good reputation for me, mostly because of a combination of properties which came together at that time.