Bungie's original Halo, released for the Xbox in 2001, was a landmark console game. Aside from giving Microsoft's freshman entry into the console arena a system seller and a uniquely Xbox cultural character, Halo was the best first-person shooter to be released on a console since the days of Goldeneye and Perfect Dark. It had intelligent single-player gameplay consisting of varied enemy encounters in open terrain, solid gunplay, support for 4-16 player local multiplayer, and a perfect control scheme and input response. When Halo 2 was released three years later (with an astonishing increase in visual fidelity over Halo), the campaign remained largely the same but the multiplayer took advantage of Xbox Live and quickly became the multiplayer game of the console generation.
Halo 3's predecessors made for a pretty rough act to follow. Aside from being the first Halo game on a new generation of consoles, what could Halo offer to the series that would have the same gravity as Halo 1's general existence and Halo 2's standard-setting multiplayer? The non-ending, second game in a trilogy sort of ending that Halo 2 had didn't really leave Halo 3 much room as far as story and game universe goes; Halo 3 had to continue the saga of Master Chief, the Covenant, the Flood, the Brutes, and so on. As such, the single-player campaign for Halo 3 was left to gamers' minds as a foregone conclusion: there will be more Master Chief, the loathsome Flood would have to come back, something about Cortana, the come-uppance that the Prophet of Truth has coming, and all of those other story threads that exist within the Halo universe as established by the first two games.
And that's exactly what happened. Halo 3 is the kind of game that everyone expects to be excellent, polished, and all-around amazingly-crafted game experience. And it is.