Recently have come back to Serious Sam First Encounter thanks to the wonderful concept of HD remakes.  The sad thing is I'm having more fun with it than I've had with any of the current FPSs coming out.  Granted my reasoning is subjective, but I've come to a couple reasons why. 
Firstly, there's little effort to make an epic story the forefront of the gameplay.  It's pure 'kill shit running at you', which results in pure FPS skill gameplay.  With this, it's also not following the route most FPSs seem to go, where they're making an effort to scare you, so you don't have all the downtime to try and create a level of suspense.  If there's any downtime here, it's because you're reloading your arsenal for the next army to mow through. 
This hasn't come with a consequence though.  Most memorably (as I just played the part tonight), the Metropolis gets extremely grinding on your interest.  The whole level is just about throwing an absurd amount of enemies in waves, and it feels like it goes on for hours and by the end of the level you just want it to end.  This leads to a bit of repetition that make you a little more anxious for the ending.  That said, they do make an effort to change up the scenery a little (though at some points not enough) to give you a sense of progression. 
 
I guess the ending sensation I get out of the whole thing is maybe to have the lack of epic storyline, but keep that epic quality setting for wrecking up  whole lotta dudes.