Gamestyle gives Halo 3 a perfect score, but that doesn't mean it's a perfect game:

The craftsmanship here is astounding, but unfortunately the game is not perfect, as there are a couple of shortfalls. The first is the game's storyline, which throws players in at the deep-end and requires prior knowledge in order to fully appreciate it. This isn't a problem for fans, but anyone else who wants to enjoy the game will be wondering what is going on.

Call me crazy, guys, but it is a sequel. There were two other games. Expecting to know exactly what's going on in Halo 3 without having either played the other two, or at least done a bit of reading, is like expecting to walk into a showing of The Two Towers and know exactly what's going on without having seen the first two films. It's nuts.