I would actually say there's a number of issues with this:
Twitch shooters are more prominent than ever - Modern Warfare and its ilk. The problem for most is that they've been homogenized into the military shooter field, or 3rd person cover shooters. Simply because it gives the appearance of more going on in the games.Look at twitch shooters (Quake and Unreal) they are no longer as popular as they once were, despite them being awesome games (to me even better then half life due to the fast paced action).
I disagree with that completely, the best FPS games don't require nonsense mechanics such as hammer X to open door, or do this quick circuit board puzzle to make them more interesting. The sheer fact that most modern shooters don't even allow you to have more than a few weapons at a time still irks me. Painkiller still stands the test of time in the department of 'pure FPS'. The problem with most modern FPS games is that they simply aren't entertaining enough in how the gunplay feels or plays out. They tend to require secondary features or systems, because they would feel hollow or dull without them. I would actually instead suggest that most games, rather than evolving, have dumbed down the First Person RPG formula significantly. Just compare System Shock 1/2 or Deus Ex to any of the quasi-FPS-RPGs you can think of.Hence why I made this thread, I respect Half life, but I do not consider it the best to date, simply because the genre evolved, it's no longer enough to give you a few guns and you go shoot stuff.
I would argue that if a FPS requires gimmicky supplements to its core gameplay, just to stop you from becoming bored, it's probably doing it wrong. There was a tangible sense of satisfaction in simply killing enemies in Half Life. Killing them more often than not had a purpose and meant you could progress. The location of both the player and enemies was relevant to the map design and (most of the time) wasn't just random, endless respawns until you moved forwards to the next breachable door. Each kill meant something. It wasn't about just getting another irrelevant achievement.
The game also rewarded exploration, but didn't demand it. It simply had areas that you could venture into at your own risk, with potential self-contained stories (expressed in the form of bodies leading to a key location, or perhaps simply a blood trail leading to an alien). Which leads me to the unique way it told the majority of its stories: It would simply lead you through an area and allow you to see what had happened, or have things happen around you, leaving you to figure out the implications or possibilities of each scenario encountered.
The game was ground breaking in numerous ways, not to mention the AI on the soldiers and even things like the huge alien in the rocket silo. It gave a sense of scale and involvement in the game world, that both made you the central focus as well as an observer who got drawn into the conflict.
All in all, these are things that other games have built on, but have yet to revolutionize in any significant way.