Well...
1- The weapons are different from anything you'll find in any other game. Semi-automatic is far superior to fully automatic, in my opinion.
2- Combat unlike what they do in most realistic shooters. You can take a few hits, and so can your enemies. Gives you a chance to fire off one-liners.
3- Beautiful settings, armor, basically everything is beautiful. And colorful.
4- Customizable armor, and two different species to choose from.
5- Vehicle combat that doesn't suck. Except tanks beat everything, so never be without one.
6- Science fiction storyline that's pretty decent, as far as I'm concerned.
7- Awesome music.
8- Hilarious moments, especially the easter eggs and IWHBYD skull.
9- Interesting and infinitely replayable campaign, with skulls that force you to change your playing style with every configuration.
10- Horde clone that doesn't suck, and doesn't get too damned hard after five minutes.
11- Gravity Hammers. And Spartan lasers. They deserve their own number.
12- Assasinations. Doesn't matter which game, it's always cool to ninja someone.
13- Enemies whose sole purpose is comic relief. I have a big grunty thirst.
14- Cool expanded universe, with all those books and stuff.
15- They were overhyped, so all my xbox friends were playing it together on launch day.
16- The beta was amazing.
17- Great stat tracking on Bungie.net
18- Screenshots and film clips.
19- It like, totally invented the first person shooter. (Just kidding, but it was the first game I played on my xbox).
20- Sergeant Avery Johnson. He knows what the ladies like.