Okay, I'm a little ways into the single-player campaign now and have played some multi-player. This is my opinion so far.
I think this is actually really a very nice shooter. I feel that mostly it deserves high ratings. Games getting 10/10 and stuff will always be controversial, but I can easily see this competing with PC giants of the genre.
I mean... interesting world with some depth, check. Nice graphics, check. Solid gameplay, check. Good story, check. Decent balance, check. Good pacing and story progression, check. Good musical score, check. Great voice acting, check. Amazing multi-player, check. Lots kick-ass brutal action scenes with huge enemies, double check. The scarab tanks are fantastic in this iteration of Halo.
Sure it has its flaws, but having seen the good parts so far, I think the people calling it only a so-so average shooter are the same sort of people who had this reaction to Bioshock as anti-hype reflex... or, maybe people are just having different opinions.