They're nothing alike but COD has a crap story and it mostly played online. Half life 2 is a masterpiece and not made for idiot teens who just want something to shoot.
Well Valve is coming out with a new engine that'd be a perfect opportunity to make Half Life 3. I really don't care if they make more Left 4 Dead games. I liked the second one but it's really nothing special. http://www.ign.com/articles/2012/11/12/valve-working-on-new-game-engine
You don't exactly give Half-Life 2 much of a chance, Half-Life2 was was made back in the late 90s- early 2000s, while CoD:BOPs2 was released not even a year ago.