Making a quantum leap in a genre is a pretty hard thing to do. The FPS genre has been full of significant technological accomplishments, but these have come to be expected by each iteration of the game. Either you build on top of a game engine or you develop technology. With that being said, I think the game that supplied the biggest leap forward in the genre is without question, Half-Life. While still built with existing tech, the gameplay and more importantly the story set the game apart from any other like it before it. Half life utilized the first person perspective to drive it forward from just mindless killing and allowed it to become mindless killing with a story context. Given the lineage of FPS games before it, I'd say that's a pretty big push forward.
-Dustin Clingman, Zeitgeist Games, Inc.
Half-life was monumental in its story telling and first-person cinematography.
Half Life was such a great leap forward in game design that games are still being made today that have yet to catch up. Notable innovations:
The graphics are the only thing this game might lack versus another FPS launched today. Gameplay wise, it stands with anything I've seen released recently.
For me, Half Life made the biggest 'quantum leap' in the FPS genre with its inclusion of story-driven gameplay, which was executed superbly.
-Jools Watsham, KingsIsle Entertainment, Inc.
Before Valve's Marc Laidlaw introduced us to Gordon Freeman, the genre remained mired in the same "no plot to get in the way of the story" that marked the genre since Wolfenstein 3D and Doom. Half-Life drew us in with characters that fascinated, leaving us begging for a sequel -- not just for more great gameplay, but for a continuation of the story. No preceeding FPS had that, and the influence of that innovation can be seen in current titles like Prey and F.E.A.R..
-Christopher Dellario, WhatIF Productions LLC
By folding in story and adventure elements, [Half-Life] created a much deeper, more immersive experience than the pure shoot-em-ups that preceded it, a you-are-there action/horror experience that pulled as much from Aliens and Day of the Dead as it did Doom and Wolfenstein. And in doing so, it set a template for almost every FPS title in the future, from Deus Ex to Halo.
-Jordan Itkowitz, Rainbow Studios
[Half-Life] is the first time I can ever recall a 3rd/1st person view game where the cutscene unfolds "around" the player rather than infront of them as most games do. It also obviously was using fully realtime ingame scenes (using the game engine) rather than pre-rendered video. Even today many of the biggest titles still do not do either of these, which is a shame as the level of immersion is way beyond that of mere pre-rendered cut scenes, that are non-interactive.
Think of the intro of Half-Life, being in the train/cart, able to look around as much as you want, while being "moved" to your starting position. It is a image I'll never forget. Half-Life 2 carries on the legacy (introduction unfolds around you at the train station) but it can not compete against the impression that Half-Life gave me! BINK video and similar has served games well, but it is time for the game engines to do what they do best, immerse the player into the game world. Half-Life did this 8 years ago (which is like 40 years in gaming years), so in my eyes that makes it a pioneer in story telling for FPS games (as well as 3rd person view games).
Half-life, mainly beacuse of its movie-like atmosphere and the fact that you are just an ordinary scientist (albeit, Rambo-like!). The engine also provided way for ordinary people to create different and beautiful mods such as Counter-Strike, which became a game on its own, They Hunger, etc. Although they are not related to Half-life (they are just mods), the fact that anyone with a little programming skill could write total conversions created a large modding community and a large user base who continued to buy Half-life years after the game had been released.
-Toni Petrina, Cyprox Studios
Although it may have not been the first title to do so, Half Life certainly was one of the best examples of narrative in a FPS. By not breaking the perspective with cutscenes, Valve crafted a game where I, at least, really felt some involvement within the story, which didn't feel like some excuse to run down a corridor to shoot enemies. Even FPS games today that rely heavily on that run and gun type of gameplay (Prey, Black) would seem primitive now without some form interactive narative or experience during gameplay.
-Blaine Toderian, Veda Games