To be honest the single player was nothing revolutionary (unlike the first Halo) but well polished and enjoyable. It was a little on the short side for me beating it in about about six hours, but most of my time with the game is with multiplayer anyway so I wasn't too annoyed. What does get annoying is that People expect an entirly fresh and origional game when the first in the series was just that. Therefore, anyone knocking it down for the sake of origionality must remember that the game was a re-birth to the genre with the rechargable health bar and weapon carrying system, so in some way or another, almost every FPS you see today builds upon some things Halo started (COD4 included).
On the other hand you have the online play. It brings two new things to the table those being forge; a map editor, and the theator which allows you to edit save and replay games and videos. Now I havn't played COD4 but I'm assuming it lacks these features which I have personally enjoyed a ton so from a community and feature standpoint I would have to say Halo leads the market undeniably.
