Most everybody likes Halo, but not everybody loves it. They don't have to. Some find things they enjoy, and ignore the rest, like Kotaku's Brian Crecente:

While Halo 3 doesn't reinvent the genre, it doesn't need to. What it does instead is provide fans of the trilogy a sort of satisfying ending and a much more satisfying experience.

I've always held that one of the things that attracts fans of Halo to the game is its plot and characters. While Halo 3 does deliver on its promise to wrap things up, I found the plot a bit too convoluted to either follow or really care about. Sure, I wanted to see what happens, how it all ends, but that's not what drove me to complete the game. Instead the experience of playing the game was my main motivator for beating it.

When pressed, Crecente commented that he'd rate Halo 3 a B.