It seriously is.
I'm not really sure why I didn't see it before. I'm guessing it was because all these years before, I'd been trying to play Halo like it was a different game. I grew up on PC first-person shooters predominantly based on the Quake III and Unreal Tournament engines, so I had no idea how to play a game that only let you carry two weapons at a time or didn't involve you merely running at enemies with guns blazing. Halo has a sort of ultra-simplistic chess-like quality. You've got a limited amount of ammo for only two weapons (plus grenades) and you have to decide how best to use your skills. Prioritization is the key; dividing up your various skills between your opponents all while paying attention to your recharging shield meter.
It struck me last year that Halo isn't a shooter in the normal sense of the word; it has an ultra-refined simplicity that one sees generally only in classic games like Mario, Zelda, and the like. Once I understood that, I went back to Halo: Combat Evolved (the first game, for people who live under rocks) and played it through. It's now one of my favorite games of all time. It's like it finally clicked in my head, and now I understand what the rest of the world saw that I didn't. I absolutely love it.
Concurrently with my Halo game-playing, I've been reading a ton of the side material for the series, mostly consisting of novels and comics.
The novels are wonderful. They take characters that seem shallow in the games and give them depth. They give the universe detail and texture. They just make everything better. The comics are less consistent. It's like no one is really doing any quality control or trying to make everything in the comics congruous with the rest of the universe.
Halo is a kind of special event. Its fans treat it with the same kind of reverence and love usually bequeathed upon movies like Star Wars. Playing Halo with friends is like playing a backyard sport. It's just for fun, but there's definitely a degree of skill to be gained and utilized effectively. It's got a perfectly-balanced focus on casual fun and competition, so no matter who you are or what you want to do with the game, you can enjoy it. That's something that other games can't seem to get quite right. Call of Duty seems to lean way too far towards the competitive side, while some other games just aren't fun for more skilled players.
There's one more facet of Halo that truly makes it special: the music. Martin "Marty" O'Donnell does much the same for Halo that John Williams did for Star Wars decades ago. His music breathes life into the universe and stands alongside the greatest soundtracks of all time—gaming or otherwise.
(skip to 0:53 and turn your sound WAY up)
To your ears:
You are most welcome.