Oh, to clarify my earlier comment. I meant that playing the campaign solo never really interested me, but I absolutely love it in co-op. Many happy memories of working through Halo 1 and 2 on legendary, sat right next to my friend.
I actually fell in love with Halo 3's multiplayer rather quickly, considering I was never really into online shooters before. Reach changed everything and ruined it. At least it looks like 4 ditched the armour-lock, though that was far from the only problem.