Serious Sam is probably my favourite recent FPS game, well over any Unreal, Halflife or Quake games, Simply because it feels fresh, exciting, and very much like the original Doom games in its relentless speed and enemy count
Raven's right, there is impressive usage of LOD - the closer you are to a model, the more detail is added - interestingly, LOD is also used on the textures, when you are flat against a wall there is little or no pixallation, (very high texture res) but as you get further back it swaps it for a smaller texture. It constantly writes and re-writes memory to keep the pace high
The thing that really slows down the gameplay in a FPS is the AI of the enemies, however in Serious Sam they are not at all intelligent. They have a basic "If player 1 comes withing 15 feet of me, run at him screaming, If player is more than 15ft away shoot at him, If player is more than 100 ft away I can't see him" kind of logic. Its hardly a thinking mans game, but thats the fun of it. When you have 30+ enemies runnin at ya at full pelt, you don't worry about AI
