Great graphics, great gameplay, great characters, great dialogue, and a great storyline! AN ABSOLUTE MUST PLAY!!!
What initially drew me in was the sci-fi story. I'm a big sci-fi/comic geek, and the story in Halo 2 is as good as any sci-fi movie. The cut-scenes are well-played by the actors (some great one-liners by The Sarge!), the cinematics look beautiful, and soundtrack is fan-freaking-tastic (thanks to a full orcestra and hard-rock band Breaking Benjamin).
As for the gameplay, this is what gaming is all about. Smooth responsive controls, an environment that is sci-fi yet feels realistic, the enemy AI responds quite well (depending on what difficulty level you pick), the fighting is intense, and environments are beautiful to admire in the few moments of rest you do get in the game.
I go back and play through this game pretty often actually. It's replayability is a 10, which is rare in modern games. That alone makes it worth buying, and at this point you kind find it at a used video game store for like $15 only. This game still stands beside any next-gen game and blows most of them out of the water. If you haven't played Halo 2, get $15 and get it NOW!