Filled with the code of Doom
I think there's no single factor. The most important thing to say is that it's a classic game in every sense of the term: it regularly and rightly features in "top games of all time" lists, and that notoriety is enough to keep up interest in it today. I think it has aged well, and it's the kind of game that I think even younger players raised on more modern FPSes can probably pick up and appreciate.
So I would like to know you are still interested in the game and why you think it is still popular.
In terms of gameplay, Doom essentially defined the modern first person shooter genre. This graph is a good example of what I mean. There will be some people who at this point will rush to correct me by pointing out that Wolfenstein 3D came first. But in some sense I feel like Wolfenstein 3D was a prototype for the FPS genre, and it was Doom that actually defined it.
There's something Doom has that I feel like Wolf3D doesn't - part of it is almost certainly the technology, but there are also thematic and artistic differences between the two. Wolf3D still had a somewhat tongue-in-cheek cartoony feel to it, while Doom brought a focus on dark themes and gritty realism. It's something that's influenced the entire industry and everything that's followed it.
Aside from its historic significance, what has kept interest in Doom alive until today?
- Point number one, and this can't really be overstated, is the fact that the source code was released. I'm fairly confident that without the Doom source code there wouldn't be a Doom community at this point. Aside from the fact that Vanilla Doom doesn't even run properly on modern machines, what you can do with it is severely limited. Having the source opens so many different doors.
- Modding is probably the second most important thing. Historically speaking, modding was one of the big features that drove interest in Doom from its very start. Before Doom there weren't really games that were designed to be modded. Instead, Id basically encouraged it and the end result were thousands of add-on levels and a kind of mini-industry that sprang up around the game. That kind of momentum has manage to continue to this day.
Doom has retained an active modding community, and again, this is fuelled by the existence of source ports that expand the range of things that modders can do. Mods like Urban Brawl demonstrate the ingenuity of modders and the kinds of things that source ports are capable of.
One interesting thing to point out is that it's really really easy to make a Doom level, even something moderately complex. The 2.5D aspect to the engine means that you can sketch out an idea for a map on a piece of paper. Think about the fact that a level for a modern game could take a professional level author months of work to complete. By contrast, the Doom community has speedmapping contests where people put together a complete set of 32 levels in 24 hours. It's my theory that Doom level editing occupies a kind of "sweet spot" where you can make something that's architecturally interesting without the effort required from a full 3D engine.
- Multiplayer is the final big thing. This is something else that Doom has had from the start - from being one of the first popular PC games to include multiplayer support, to pioneering Internet and online gameplay (DWANGO). Doom has retained a somewhat-separate community of deathmatch players. This is again something that has been fueled by source ports: without ports like csDoom I don't think there would be so much interest.
Doom's deathmatch dynamic is different to more recent games, which I think has kept some interest. Things like the player speed and weapon selection make for a much faster-paced game that other games haven't really reproduced.
- Perhaps it's only a minor thing really, but since its release in 1993, it has always been available. Id have kept it going with numerous new releases and re-releases: early on with the Ultimate Doom and Final Doom, then with boxed sets like the Depth of Doom trilogy and the budget Collector's Edition CD that I even saw for sale in my local supermarket. It's been available for download via the Id website for years and now via Steam as well. More recently we've seen Doom RPG, the XBLA version, the iPhone version and now the BFG Edition version. Unlike other games it's not just a forgotten historical relic: it's still around, still for sale, new versions are still being released.