Prior to playing Black Ops on multiplayer, I found the campaigns in the CoD games to be easily the best part. And if you go through and actually play the campaign in Black Ops, which I have a feeling you didn't, you will see just how excellently made it was.
If you ask me, they should take a few hints from Halo, and allow co-op into their campaign. Just the same, allow more customization into the campaign mode as well, also like Halo.
Furthermore, I believe that they should in fact devote a lot more time into optimizing the single player experience of the CoD series, rather than focusing it on what is only really a -decent- multiplayer experience. Sure, the multiplayer is fun, but it is quickly ruined once Killstreak rewards, easily the dumbest possible thing they've put into the game, come into play. Not to mention other utterly ridiculous aspects of multiplayer... for instance, the Famas having almost zero recoil when spraying, the Ak74u being stupidly accurate and unbalanced, and don't even get me started on the damn knife.