I feel like I had to defend Halo's honor by making an account and stating the obvious. Halo was NOT responsible for the decline of the first-person shooter genre, even if it did, it did no more damage than the pile of shit the autists in this thread have been parading as if it were a masterpiece: Half-Life.
Half-Life is the game we should point to for starting the industry-wide trend of "cinematization" and casualization. Even the likes of Lilura pointed this out in "her" blog, about how Half-Life began the gradual decline of first-person shooters into Hollywood-inspired storyfaggotry. You know what doesn't happen in Doom, Quake and even Halo?. Unskippable cutscenes that play out during live gameplay. You know what most gaymers complain about today?, unskippable cutscenes that play out during live gameplay. It all comes back to Half-Life. And nothing about that game is even remotely noteworthy aside from that. The gunplay is decent if not average, the graphics are poor even by 1998 standards, and the game lacks ANY replayability because of it's scripted nature. Some mods allow you to select chapters/sequences of the story, but these could hardly be considered replayable out-of-context. Meanwhile you can play through any level in Doom, Quake and Halo and have a good time, because they don't tie you down to a strictly linear sequence of events.
Halo is not a perfect game by any means though. It suffers because the developers were hard pressed for time, and had to conjure up a game that played well on a controller it just eight months. But during that time, by an absolute miracle, they crafted an engine that has stood the test of time, and inspired the control scheme of every console shooter that was to come. By influence alone, Halo utterly OBLITERATES Half-Life. The only thing Half-Life did was lower the standards of gameplay in service of narrative, which is the antithesis of the entire medium. Sure you can say it works well in an RPG, but that's not the genre we're discussing here.
Had Halo been given the benefit of the PC as it's primary platform, and a proper development cycle, Lord only knows the masterpiece that could have come of it. But even as it is, it's a great game that manages to shine despite it's shortcomings, and it's gameplay was gradually perfected before reaching it's zenith in Halo 3.