What started as an ambitious source-port of the classic Doom videogame and it's derivatives (e.g., Heretic/Hexen), the Doomsday engine has evolved to one of the most impressive enhancements of the videogame both visually and in capability. I'm not sure of the exact specifics of the codebase, but whether or not it actually is true the Doomsday engine seems like a complete engine re-write, supporting many modern FPS features and a highly optimizied graphics engine.
There is just too much to say about it, it has near capability to power current-gen games and is overlooked too often by hobbyist/independant game creators. I can however one day see Doomsday maturing into a full-scale engine independant of Doom-esque videogames. So any of you game creators out there reading this, take a look at what this can do - and consider the power of Doomsday for your next game or 3D interactive software idea.
+ Extremely impressive graphical engine
+ Very well designed and layed-out (user interface and frontend wise)
+ Very easily programmable with existing tools (for example, map-making with pretty much any Doom WAD or BSP map maker)
+ Uses common fileformats for data and resources, no proprietary garbage (plain text definitions/scripts, MD2 models, OGG and MP3 audio, etc)
+ FAST as hell!
- No physics support (yet)
- Limited multiplayer capabilities compared to Skulltag for example (but this is being improved!)
- Lack of detailed documentation for "mod-from-scratch" developers (to be expected with such a dynamic project)