I pretty much agree with you on most respects. I'm not even too keen on Halo's controls, plus, if you think Halo is the greatest FPS out there, then you have not played Half-Life 2 or Crysis, which (IMO) trumps Halo 3 on practically every level.
However, look at what Halo has done for XBOX. It was the game that sold XBOX consoles, just as Mario did for Nintendo. It's practically their mascot, but with less spin-offs. Halo 2 was an outstanding online game for consoles, probably the best Microsoft had to offer on the XBOX, and certainly better than any Sony online game (Gamecube was sort of mute, unless you count PS Online). So naturally, hype about the 3rd game in the series was going to sky-rocket sales to a ridiculous extent.
I, for one, loved the story and the mythology behind the games. I've read the books, and they were very well-written, especially Fall of Reach. Immersing yourself into that world is probably the one thing that turned most actual gamers on for the last one, because it's the endgame of everything. MC's story will finish, the Covenant will finish, the Halo weapons will be finished. Everything resolves. That's why people wanted to play it. They wanted to know the answer.
I don't even own the game or Halo 2, but it's EXTREMELY pretty and fun with someone else. But when I look at Halo 3, I don't really look at just the game. I look at the story behind it, because let's face it, games are becoming more and more like interactive movies these days. And the story is what dragged me in. A rogue hero, the last of his kind, fighting not only for the glory of his fallen comrades and his people, but struggling to find out exactly who he is and what his true purpose is.
And that is why I think Halo is such a great game.