I’ve been playing shooters since Wolfensgein came on floppy disks, and Halo was exceptional.
Bungie nailed the combat loop. Each little firefight could play out different each time you play, depending on your tactics, and the AI response. Speaking of which, the AI was pretty darn good, giving each Covenenant species a distinct “personality” in combat, coupled with the voice-acting, which enhanced that feeling. (There’s a reason why they record dozens upon dozens of different lines for each enemy).
Gameplay also reduced the weapons to two-at-a-time, when most shooters to that point were like Doom in that every weapon in your arsenal is always on hand. Having two weapons means your constantly having to make choices about what to carry, what that further determines what tactics you’ll adopt for any encounter.
The “open world” nature of some of the levels also got away from the rigid corridor feel of many shooters. Oh, and the vehicle and infantry gameplay was damn near cutting-edge and totally seamless (again helped by the presence of friendly AI who not only gunned for you, but also provided fun commentary along the way).
And just when you’re really settling in to the covenenant, they introduce the flood, and the combat loop goes to a whole new level as now the Covies and Flood will battle one-another while you’re running around causing havoc.
Then there’s the finale, which had no boss battle but an insane driving sequence that was absolutely hilarious and thrilling the first time you played.
Layer on Bungie’s sense of univer-building and character (the latter almost totally provided by Cortana as chief barely speaks). And it’s easily a Hall of Fame game.
And that’s not even mentioning the multiplayer, which allows split-screen co-op and LAN parties. Both of which were social and fun as hell, and there’s a reason guys will post photos of old-school Halo LAN parties to reddit all the time. Hanging out with your best friends all night eating pizza and playing Halo remains a seminal moment for many.