One of the all-time great First Person Shooters
Call of Duty: Modern Warfare 2 did one of the rarest things a video game can do. While being a direct sequel to a masterpiece and revolutionary game, was albe to keep the same amazing level and even improve the formula in some aspects. It's one of the best first person shooters of all time, without question.
The game kept what Call of Duty 4 started, the multiplayer with lots of loadout options and killstreaks to choose from. They added also the deathstreak, so players that couldn't perform that well could get a boost to try and get back to the match. The amount of weapons was still amazing, as well as the amount of stuff you could attach into them. They improved on the maps design as well, giving some fresh looks that became classics of the franchise.
Alot of people complain that the singleplayer campaing is too short, but in fact I liked it, because if feels straight to the point, just cool missions, with the important stuff, without any unnecessary time filler to just make it bulkier. It's a really good revenge story, who takes place a few years after the campaing of the first Modern Warfare game.
The game also has the Spec Ops mode, where you can play a set of missions, co-op with a friend. With some really nice variety and cool ideas, mostly being twists from moments in the singleplayer campaing. Sometimes one player will be on foot being cover by the other player from somewhere in the air. There's the more traditional wave cleaning. Some really cool stuff, that rewards you with stars so that you can unlock more levels and keep progressing.
It was the Call of Duty game with the most fun content I can remember, and the highest peek of the multiplayer gameplay loop. Keeping and improving the formula of a revolutionary game. With more balanced weapons and better map design.