Halo is the series that many people love to hate. It is undeniably a very popular series, and when other gamers, particularly those inclined to PC FPS decide to see what all the fuss is about they just see a generic FPS. "Why do you like this game so much! Haven't you played any other games before?" Halo 2 and Halo 3 where your average online multilayer FPS, with a very healthy serving of options, and an decent single player... but they're no half life 2. So why do i think that halo 1, a simpler game than halo 2/3, albeit better designed, is a landmark game? Well its all about context.
The easiest to explain, and least important, is halo's graphics/technilogical achievements. Looking at screen shot of halo 1 now looks nothing special. Of course we all know its supposed to, it's an old game. So to put in perspective how good it DID look remeber it was a flag ship game for what was then a next gen console. Needless to say, it looked good. It had huge spanning levels, massive beautiful sky boxes, a sense of scale that the introduction of the hard drive to the console made possible. I remember looking straight down at a grass texture below my feet and thinking wow, im really close to that texture and it doesn't look pixely. I got the sniper with scope 10x and did the same thing, and it still didn't get pixely... now days thats standard, but back then it was quite an achievement.
Playing some of the larger single player now levels gets tedious (well just 1 in particular), but back then there was novelty just in the size of things. Having playable levels in the scale of kilometers is something that we just take for granted in games now. Taking things "For granted" is precisely what the main argument of this post.
Here is a some of the things that halo had, that are industry standard now, but were pioneered by halo.
Regenerating health - Doesn't sound like much, but it makes a very fundamental change to gameplay. It was a feature that defined halo at the time, now maybe 50% of FPSs have it. (as an interesting note, halo 1 actually had the shield-hp dynamic, where there was a layer of regenerating shield, and a final layer of hp that you needed a health pack to get back. It was the best of both styles of health in one, but it was inexplicably removed in halo 2/3 in favor of regenerating hp as well.)
Vehicle combat - The seamless transition from first person shooter to 3rd person driver was a trade mark halo feature, again now very common. It was the first FPS to ever get vehicles right. Having vehicles, in a multilayer game, that don't take the focus too far away from shooting, (it is an FPS after all) that are hard but balanced, and fun and practical to use is quite an achievement. It was a perfect balance that i don't think has been matched yet, even by halo 3, and defiantly not by halo 2. This is of course personally opinion, but what is not opinion is that it was certainly the first game to really nail this game play element.
'Secondary fire' melee/grenades - Yet again this feature absolutely defined halo 1 at the time. The ability to throw a grenade, or melee no matter what weapon you had out at the time. Having a 'button' for grenade and a 'button' for melee as apposed to switching to grenade or a melee weapon is such a simple idea you can take it for granted. Like with regenerating health it makes a fundamental difference to the way the game plays. But when halo 1 did it was new, and there was a sense of, 'why didn't anyone think of this before?' that you can forget existed when looking back now.
Console FPS - If golden eye and perfect dark showed that FPSs are possible on a console, Halo showed that they are practical. The control scheme of halo has been more or less copy and pasted into every other console FPS since (and third person shooters for that matter). If there wan an PC FPS game that invented 'mouse look', then halo is the console equivalent.