Firstly, I would like to congratulate you on starting what might turn out to be a page-long debate, it's always nice to pit people against each other.
Halo, in my opinion, isn't anything special or unique. It's very fun to play, but mirrors every cliche in the book, why do we love it? Because it shows us just how good we can make things we've already seen before, and that our best is never enough. Halo in question has the second-best multiplayer i've seen (just below Counter-Strike and just above Perfect Dark).