Zar:
I think the parallel with Doom doesn't really have a great deal to do with thousands of enemies coming at you at once. Similarities are more like:
* Simple puzzles. Find stuff. Walk around. Shoot things
* No strategy, bosses excepted. Sure, there's a bit of a trick to knowing the right weapons for the right monsters, but you won't need a Prima strategy guide to finish it
* Run a mile, hit a button, and run back a mile to places you've previously cleared but have spawned more monsters
* Frequent occasions where you are struggling to cope with the number of enemies. Even with only a dozen or so at once, Doom was really good at this. Sam enemies are fairly tame by comparison, but make up for it in numbers
* Complete lack of emphasis on plot
Also the general feel of the game bears a resemblance to Doom 2. It's hard to describe the sort of absent-minded carnage mindframe that exists in Serious Sam, but if you said, "well, it plays kind of like DOOM," people will know what you're on about.