If I could change one thing in Black Ops, I would attempt to better separate casual and hardcore gamers from each other. One way would be to offer them different contracts.
The current system makes all contracts available to all players. Only the most skilled players even attempt the most difficult contracts which offer the best rewards. This is silly, because more skilled players have an easier time leveling up, anyway. They also tend to be achievement-oriented gamers (challenge junkies) who would be more tolerant of a slower pace of leveling. It's the less skilled players who most appreciate and need the bonus XP and CoD Points.
So separate them.
Black Ops already allows players to sacrifice all progression for Prestige. Offer players interested in Prestige more difficult contracts. Make it so that the hardcore's "easy" contracts are similar to the casuals' "difficult" contracts.
This would lessen the frustration of casual players by giving them contracts which are both within their ability and worth plenty of XP. More accessible contracts would provide more short-term enjoyment, and more XP would increase long-term enjoyment.
Currently, too many CoD players are frustrated because they are lobbed in with players of vastly different skill levels.