Hard on it? Really? I was being nice with that...well to answer your question the first problem is the length of the single player campaign. I mentioned back when MW2 came out and with several halo games that a single player game should have at least 8-12 hours worth of gameplay to be listed at full price. My main complaints go to the multiplayer though. Just like MW2 many of the killstreaks are just completely cheap, overpowered, or both. With people earning kill streaks so fast it equates to a broken multiplayer that, for me, is less fun then watching paint dry. Granted some things were fixed like quickscoping but so much more was left the same that I just cant have fun with the game. Its boring, its tedious, and its frustrating.
Granted maybe I expect more out of it because I play battlefield so much. In battlefield it is not a run and gun game, but at the same time it rewards real teamwork, role playing, and critical thinking. CoD doesnt have that kind of stuff, its always the same thing of being stuck on a small map, running and gunning, with all sorts of overpowered weapons, perks, or both.