Throughout the Call of Duty games, we've always had the same system over and over again. Unlock the same basic generic camos just to get to the really good ones. Nothing really changed and it was often an aggravating grind due to the same challenges over and over.
When MWII was released, they decided to change things up. Rather than going through the same camo unlocks, every weapon had their unique camos with different ways to unlock them. Sure, the path involved was definitely shorter, but I found it way more unique and efficient.
It also motivated me to use different weapons. If I saw that a certain weapon had an awesome camo and it was locked with a sniper rifle challenge, I was actually happy to use that weapon despite never using it regularly. As long as I acquired something that I wanted, I was happy.
I really hope it comes back. Another idea was having objective XP directly tie with camo unlocks, but that will probably never happen.