The classic FPS game Doom has turned 20 this year. This classic game that first came out in 1993 has lasted all this time and is now even more popular. 20 years of creating maps and blasting monsters sure has changed the world for the better. The first version of Doom was uploaded onto FTP sites in 1993 and since then gaming was changed forever. I wish that more of the features that were in the alpha versions had made it into the final release, but dark creepy maps such as E2M6 are the hallmark of Doom. The ability to have textures mapped to walls and lighting levels allows the creepiness factor of your Doom levels to be jacked up to eleven. Another feature of Doom is the ability to perform various tricks, sometimes they are unintended: http://www.doomworld.com/vb/doom-speed-demos/35457-ultimate-doom-trick/. But most of the time they are intentional. Like pushing up against a metal fence texture and your character making it through. This is used in Doom2 MAP10.
But most of the time we are just playing the maps properly and killing monsters. That is where all of the fun comes from. Doom maps can have a massive amount of monsters. Nuts3 has over 12,000 monsters in one huge area. This map I made: http://www.securitronlinux.com/files/aa1.rar has over 430,000 monsters, but is not very playable on a current PC. Modern FPS games have enormous maps, but not that many characters in a map at once. The modern Gzdoom and 3Dge source ports have support for creating solid 3D maps with bridges that you can walk over and under. That is a far cry from the relatively flat maps in classic Doom. But since the first episode has the best level design ever seen in a computer game, who is complaining about that? E1M1 Computer Station is one of the best game levels ever. Hopefully this game will be around for the 40th anniversary of Doom. There would be incredible Doom source ports by then.
The old Doom engine from 1993 had a few quirks that could be exploited to clip through barriers and get to places you ordinarily could not reach, or to get through a map faster than normal. This is used by speedrunners that seek to get through a map as fast as possible. Tricks such as running at an angle with the run key held down to get as much speed as possible are very useful. Sure, modern Doom is very nice, but it can not have as many monsters as the classic game. Okupluk slaughter map has 23,000 monsters in MAP05. That is fun when played with Russian Overkill. If you tried that wad with Brutal Doom, it would probably melt your computer.
But it is fun mowing through the demons with insanely overpowered weaons like the Yamoto Cannon. That can wipe out a 1000 Cyberdemons with one shot. Heaps of fun.