Fixing What Shouldn't Have Been Broken.
Single-player: boring. It's the same thing, again, with new locales. Although the story is engaging, it's not worth the linear levels that barely reach acceptable standards in today's FPS games. With campaigns now that are non-linear, far more action-packed, and less "YOU ARE THE TERMINATOR!" styles, I feel they could have done better.
The MP though, is where Black Ops shines bright. They have brought back dedicated servers, lean, got rid of nukes, Stopping Power, Juggernaut/Painkiller, as well as a lot of cheap weapons. The balance between the different classes is significantly better this time around, and the maps are less kill-box like and more open ended.
Rather than completing challenges, you now purchase upgrades for your gun, which lets you customize a gun the way you like right off the bat. You purchase weapons, perks, attachments, reticule items, playercard changes, and much more through the COD points given out to everyone after each game. The better you do, the more you get, but you don't necessarily need to do well in order to have those COD points sitting there.
Overall, Black Ops is a fresh start for the series, but you can definitely see Activision's overbearing presence in the game, which has led to yet another rushed Call of Duty game.