Seth Carter said:
Its the codifying point of the cinematic or story driven FPS. So depending on your overview, it either ruined the genre irrevocably, or pulled it into the modern era (or more moderately, added an optional branch that became unfortunately over-dominant)
Essentially, as the FPS genre developed in the mid-late 90s several subgenres emerged from the previous baseline arcaey shooter style (Doom, et al)
Half-life(1998) was the more story-driven or cinematic effort. With NPCs, and cutscenes/background events and so on.
System Shock (1994) was the early-FPS RPG thing.
Thief (1998) was the stealth branch.
Unreal Tournament (1999) was the multiplayer-focused shooter.
Hexen (1995) was the hero-shooter.
Rainbow Six (1998) for your tactical squad shooter-sim.
The original Team Fortress also merits a mention, it was a popular mod for Quake(1996) as the less simulation-style squad-based stuff goes.
You're missing a few, but just off the top of my head:
Before Wolfenstein 3D, there was no first-person gun wielding free moving on-foot shooter.
Before Doom there was no network multiplayer
Before Heretic, no FPS has Y axis aiming or gibs
Before Marathon, FPS didn't have objectives, beyond 'kill all enemies' and 'find key to the next place', and they didn't have friendly NPCs, mouse free look, or dual wielding
Before Dark Forces, FPS didn't mix action with puzzles (as Half-Life later would)
Before Quake, there was no full 3D FPS
Before Duke Nukem 3D, FPS didn't have non-linear levels designed as the buildings they represent rather than a corridor to funnel the player through
Before Goldeneye, no FPS had adjustable zoomable sniper scopes, NPCs that fought alongside you, location based body damage, different objectives for different difficulties, all dialogue presented to you in 1st person during gameplay, stealth mechanics, and civilians
Before Turok 2, no FPS had a weapon wheel
Before Unreal, no FPS built levels as a seamless, interconnected construct without pause inbetween...
Come to think of it... Half Life didn't really do
anything that hadn't been done before. The only difference was that it did a lot of it together and with style. Sometimes that's all you need to get a surefire hit, I guess