I dont think they missed the mark at all. The single player is basically the same as the first, which was to be expected frankly, with somewhat prettier graphics but the same game physics. The cool part they added to single player is the fact you get to play both Covenant and Master Chief, which I liked. They also set things up for Halo 3 pretty well from what I heard.
The BIGGEST enhancement to this game is its XBOX LIVE playability, which totally owns any other XBOX game out there today, and rivals many PC games. Multiplayer is a blast, easy to do, and with online stats, custom clan tags, and other nice additions, the online part of this game by FAR outshines Halo 1.
Of course, if you only bought the game for single player, then yea I guess it could be a letdown, but Bungie was pretty clear with where their focus was for this installment of the Halo story.