I hate how childishly polarizing games like Halo are with some people, so I seldom discuss it or the series, but at least it's decent so far in this thread.
And the snobby PC elitism, let's get that out of the way ("Hurr Durr PC FPS were 70,000x better and Halo was for n00bs omg lol). I'd been playing FPS since the shareware Wolfenstein 3D back on the family 386. I still loved Halo.
And the inevitable exdeath "I hate the culture it spawned" nonsense is irrelevant and entirely unfair for judging the game on its own merits, not its legacy, so that needs to be dispelled.
It's more than just a sum of its parts. Yes, even if those parts individually had been done before.
Halo was a great game because of all the little things it did right.
+ An excellent orchestral score
+ Wide open areas and varied environments,
+A compelling story with likable characters (in an FPS!)
+The best console FPS controls to date
+Very good AI
+Stellar graphics and visuals with a consistent framerate
+Satisfying weapons
+Previously unheard of vehicle driving/flying possibilities in a story-driven FPS
+Excellent, unscripted AI companions that actually made you feel a part of a larger war
+One of the best co-op experiences to that point, of any genre, period
+Fun Deathmatch and competitive multiplayer
..to name a few. The co-op especially was fantastic. Playing split-screen with a buddy next to you, driving the Warthog like a madman while he tries to aim the cannon on the back, shouting quick strategies and warnings to each other in the heat of combat with your AI soldiers fighting alongside was just damn fun.
Of course the control was woefully inferior to PC and KB+m, and still is, and likely always will be. That goes without saying. But as a game as a whole those experiences it offered were top-notch.