I feel like the new games really let the old FPSes come into their own. There was only so much you could do with the old engines, now the differences are more than aesthetic.
Wolfenstein went from "I hate Nazis" to "Stealthy-shooty multi-pathway sniper-dual-wielding WWII game".
Doom went from "Space demons" to "MASS MURDER ALL UNCEASING BLOODSHED NEVER STOP MOVING KILL EVERYTHING I HAVE 80 GUNS FUCK YOU ALL KILL MURDER MURDER EACH LIVING BEING A SLAYER OF ALL HELL AM I".
Shadow Warrior went from "Katanas and ninja stars, but mostly sci-fi-guns like the rest of them" to "Surprisingly awesome samurai-sword-fighting simulator, with guns for the scrublords.
Duke Nukem went from "The snarky one with the jetpack, minigames, and interesting side bits" to "The one that FUCKING SUCKS".
Serious Sam went from "The Balls-to-the-wall hard, sadistically designed 3D shooter with giant monsters and hordes and environments" to "The balls-to-the-wall hard 3D shooter with giant monsters and hordes and environments".
Redneck Rampage went from "Gloriously politically incorrect slaughterfest of gore and debauchery!" to "Dead, as it probably should be..."
Never knew what happened to Blood, but given how it's suddenly seized up into nothingness, I can only assume it's been tied up in copyright shit.