The Ultimate Halo
The daily and weekly challenges alone offer a ton of ongoing replay value. These are tasks that Bungie sets out every day (and week) for gamers to shoot for with a credit prize for completing the goal. These range from killing a set number of enemies in any game mode to completing a campaign mission on a specific difficulty with a specific set of skulls on to tallying a set number of assists in a specific game mode and on and on and on.
The single player campaign is challenging, but achievable at the top difficulty and can be scaled down to a level anyone can enjoy at the lower difficulty settings.
The multiplayer is the pinnacle of online gaming in my opinion and as if past Halo games are any indication we should enjoy multiple years of a lovingly refined online experience. Halo Reach jumped out of the gate with great multiplayer and I can't wait to see how things morph as time passes.