Ground Breaking
The campaign is just plain fun. You travel throughout mostly open outdoor levels with some indoor moments (with the exception of about 3 levels) in search of an ancient Halo artifact. The co-op mode of the campaign is what me and my roommate absolutely wore out. It provides hours upon hours of endless fun.
The multiplayer mode is where this game really breaks out of its shell. It can best be described as revolutionary. It is unbelievably addicting and gave me and my friends something to do at all times over the summer. We still get together and LAN it up old school with the original Halo (even though Halo 2 has been out for quite some time now). I've yet to stay up all night playing Halo 2 like I did countless times with the original Halo (and that's not to take away anything from Halo 2). It's hard to beat a game of Rockets on Prisoner, Team Slayer at Hang'em High or Snipers on Boarding Action .... Those are some things that Halo 2 can't touch and that will always separate the original Halo from its big brother.