Modern Warfare 3. MW2 and Black Ops(1 and 2) may not have been anything speical, but MW3 was just... just bad. MW2 could at least be fun at times, but MW3 just made me really ask why people keep buying these games.
Nothing. It's basically the same game with some minor changes to weapons and slightly updated graphics.
Though I preferred CSS's look over GO's and general feel, it's pretty easy to see not much has actually changed. It's still Counter-Strike, people are just complaining because it's new. Just like people to CSS when it came out.