Call of Duty lost it's charm and creativity the moment it decided to come out with yearly installments. I loved the original and it's expansion, having some of my greatest online gaming moments with them. I loved the story and the action presented. It was all great.
The second one was just as good! I didn't enjoy the multiplayer quite as much, as it cut back the large expansive maps and servers and staretd funnelling everything down to a smaller area, but it was still fun.
I don't even regard Call of Duty 3 as a true Call of Duty game. It was just so... mediocre as compared to its predecessors.
Call of Duty 4: Modern Warfare was the pinnacle, setting the ball rolling for modern day shooters. It was pretty, it was well-paced, well-balanced, had really entertaining gameplay even despite limiting the size of the maps, and was just awesome overall.
Then World at War came out. Step backwards. Then Modern Warfare 2. Step sideways. Then Black Ops. Another step back.
The only thing that's been new and exciting about any of those last three games were some of the multiplayer options you get in Black Ops, which I actually find much better than everything since the original. Wager matches are lots of fun. Most (but not all) of the maps are well-balanced. The weapons are fun. And it has the multiple-people-on-one-console thing that helped make Halo multiplayer so good too. It's good times all around.
Of course, I don't own Black Ops and refuse to spend money on it. I only know this from playing with a friend at his house.
Long story short, Call of Duty has been losing ground since the first Modern Warfare came out, except in the multiplayer department. Though that only recently started making headway once again.