Well the answer is that it's a competent shooter, the only thing truly remarkable is the hype and its the sequel to some of the greatest games ever made. Bungie has one of the worlds largest fanbasses, if large magazines started giving poor reviews to some peoples favorite franchises, they would loose customers: essentially, they sell out. If a game is trully bad, then they will throw the game in the mud: why I still subscribe to them, but halo 3, mass effect, COD 4 all got ten. While I have to admit, each one is a good game, its not anywhere as good as lesser hyped games. For example Fable is one of the best game I have ever played, yet they gave a one page review and a 8.9. Then Halo 2 comes out: the game is fun and all, but it has major problems, many of the game mechanics are flawed, everything is unpolsihed, glitches everywhere, lesser maps and gameplay: gets the highest score at the time. They hide behind all these flaws by saying "no game is perfect, every game has its flaws", and that is true, but you shouldnt ignore them if they are major issues. For example, one of the major (and few) improvements from halo 2 was the user frendliness: making gametypes in halo 1 was a *****. Like the pistol which offered great ballance (wow! now you arent automaticly dead when the other team gets both snipers or spawn next to someone with the rockets! SNIPPING WHORE!), they come across greatness and then decide to throw it out, reducing the user frendliness from the 90's to the negatives. Think about what history would be like if smart people used this attitude: Christopher Columbus: WOW! new continents, fertile land! rich mountains! thick forests! Wow! this land may be the future of the progressing world! But now that we've found it, lets do something completely different, lets say, circle the globe, and dont think back, tell anyone about it, or make any further attention or investment on this." Good thing Columbus didnt work for bungie.
To date, the most reliable reviewer i know of is yahtzee. I know, he does it in a humerous fashion, but he is nearly 100% right on the money each time. As of now the underdog developers are the best i thinks. Bethesda, Gearbox, Bioware. I find that these companies nearly always live up to their hype. My personal favorite is Gearbox, because companies like bungie will listen to their impatient eight year old fans and shave say, 3 months off of development and poslishing time, knowing they'll sell record numbers anyways. Gearbox is different, they need 3 or 4 extra months they'll take it, and tell the eight year olds to fuck off.
all in all, halo 3 is one of those above average games. If it was the first released, it would have gotten average sales.