Although MMORPG's had been out for PC long before the PSO, this was the first foray for consoles to take games online, and this game took that to levels that were long thought to be exclusive to PC's. It garnered a large fan base and became the precursor to what consoles games could do online.
This game brought the open world gameplay from a top down perspective to a fully realized 3D world, and with it a terrific story with a fun mission structure that other games have replicated afterwards. This game has defined what it takes to do a truly open a 3D world that can offer lots of fun for gamers.
Not the first game to define shooters, but it was the first to really show the strengths of a console shooter, with a large single player campaign and a surprisingly addictive multiplayer component that introduced a LAN system that worked for a console and once again popularized LAN parties. It also was the game that popularized the now common regenerating health.
Not the first game to be a FPS on the PC, it was the one to introduce and/or popularize the conquest Vs. mode and provide sprawling battle fields (hence the name) with vehicles that would make a player feel like they were part of an actual war amongst peers. No one has been able to really replicate this feeling in a game, aside from DICE when they release a Battlefield sequel.
This was one of the few Star Wars games that were introduced that had NOTHING to do with the chronology or took place remotely near the movie trilogy and it's prequels. This game introduced a new way of telling an epic story in the Star Wars universe, as well as make one of the biggest splashes in the RPG community. The combat was so enjoyable, the amount of options available to a player, this game is showed that Western RPG's meant business.
Not the first FPS, but it re-defined how to create an immersible shooter with lovable characters, great set pieces, and great shooting fun. Not to mention the heavy use of puzzles involving a robust physics engine made from the ground up with Source, an engine that still makes waves with each new Valve release.
Adding better graphics, new game mechanics for the franchise such as duel wielding and destructible vehicles, it also introduced an online infrastructure that has changed the way people play consoles and PC's online. With Bungie's implementation of an effective matchmaking system to pair people in games, it also introduced the now common party feature to bring friends with you through different games, just like if they were on the couch with you.
This game was an excellent breath of fresh air for an aging series, introducing a new camera perspective and gameplay style that has been replicated since. Ever since this game, many 3rd person shooters have since adopted the over shoulder camera and the contextual gameplay situations that were so deep with this game. With great action and horror style, this game was considered the best Resident Evil for good reason.
This game really showed what the Xbox360/PS3 generation of consoles could do. With a decent story and huge action, this game made the par for cover-based mechanics for a shooter. Although this game took advantage of various shooter gameplay mechanics introduced with many of the games on this list, it blended everything with its own style and made a game that has yet to be matched in its same style.
This game really defined how shooters handle the new modern age. With a insanely detailed and fluid graphics engine, this game took modern combat to levels that were attempted but never this polished. Not to mention that the multiplayer for this game redefined how you handle multiplayer and introduce incentives to the player to keep on playing. Modern Warfare's multiplayer level system has since been replicated many times and continues to be addictive to this day.
These are games that, in my view, were the ones to introduce unique gameplay mechanics and truly brought about new ideas to the gaming world that people actually use quite often today without a thought. List is ordered by year of appearance.