A question many people ask who don't play Halo is, "What's so good about it? What makes it better than any other FPS?"
There are things about Halo that other games beat it at, and there are things that not everyone enjoys about it. Which makes that question hard to answer.
I think that the answer, quite simply, would be "everything". Here's my reasons.
The campaign of the game starts off with a lone warrior. That's a bit cliche, until we learn that he wasn't always alone. That he used to be part of a team. And that most of that team is dead, and that they died defending the human race. Which is exactly what the lone warrior intends to do. You fight enemies with technology that seems to be mystical, until you actually pick it up and use it on them. You pick up the game and play it, and you find the story to be kind of... empty. Then you find out about the Haloverse, and all the books that go with the games that fill in the gaps. That's what grabbed me about Halo. I was mainly into it for the multiplayer with my friends, but then when I actually picked up one of the books I was forever hooked. Turns out the story of the games is only 1/1000th of the full story. The story about the technologically superior alien race that is determined to wipe out humanity, and humanity doing everything it can to simply survive. The story about what originally started the SPARTAN and MJOLNIR programs. The story of how Master Chief Petty Officer John Spartan 117 came to be in a cryotube aboard the Pillar of Autumn. That in and of itself sets it apart from most FPS games.
Then we have the multiplayer. The constantly evolving, adapting, and fun multiplayer. Everyone has their own favorite Halo multiplayer, and that to me is amazing. That the games can be so different from each other that people will argue for hours over whether Halo 3 or Halo Reach had the better multiplayer. The simplicity of the multiplayer at the beginning was what hooked most of us older gamers. Pressing one button to switch between two weapons at the time was revolutionary. And there were some pretty cool ways to play as well. Then things started getting a bit more complex, but even then that added more fun to the game. Customizable gametypes, the ability to play with people who you've never met before and might not again while still playing with friends, more vehicles and weapons. And then they let you customize armor, gametypes, and even maps to play on. Even people who didn't want to play multiplayer that often always had something to do. Then came Forge World and more armor customization. It was to the point where you could tell a player just by the type of armor they were wearing. You could spend hours customizing your character and building a map and gametype. And all of this on a console.
And then we have the community. Sure, it's not perfect, but it's still more tight knit than most. There are so many different people that contribute to Halo that it's unbelievable. There is enough fan fiction to fill entire novels. There's enough Machinima to last you for days (maybe weeks), even if you were watching it all back to back. And all of that is from the Halo community. That makes the game SO much more fun to me. When I can discover a new detail in the campaign and have an active discussion with a dozen people about it, that's when you know there's something good about the game. The fanbase is no more or less loyal than other games, but it's definitely more "in depth". There are people in the community who create and write backstories for their spartans, which is something you usually only see in RPG's.
There are plenty of games that can say that they have a feature that Halo does. Whether it be a good story, a good multiplayer, a good fanbase, or a good game mechanic. The thing about Halo, though, is that it has them all. Whether you play it for just the story, for just the multiplayer, or just the ability to create, you play it because you enjoy what you can do in the game.
That's the reason I think Halo is awesome, and am definitely looking forward to Halo 5.