The same great action of the previous games. Lots of multiplayer options.
Not much changed since Call of Duty: Modern Warfare 2. Single-player campaign is confusing for people who haven't played the other Modern Warfare games. Modern Warfare 3 picks up where Modern Warfare 2 left off.
I mean, exactly where it left off. Russia is at war with America, Makarov is at large, Soap's injured, and everything is blowing up. If you didn't play Modern Warfare 2, which served as a decent starting point and didn't require the first game to understand it, you won't have any idea about what's going on, and the game doesn't do much to get you up to date. It feels very much like the third movie in a trilogy, and the second movie ended on a cliffhanger.
The gameplay, both in single player and multiplayer modes, is identical to that of Modern Warfare 2. You can carry two weapons and two types of grenades (one offensive, one special) at one time and you have to hide behind cover and employ tactical savvy in your approach firefights if you want to kill your enemies. It's an established and functional style that captures modern, "realistic" first-person shooter games. If you get hit a few times, you need to hide behind cover until you heal, or a few more stray bullets can kill you. It's unforgiving, but balanced.
Multiplayer is the big draw of Modern Warfare 3, and it pays off just as much as it did in the other games in the series. There are the same seemingly endless customization options, with an experience system and the ability to build your own class with different weapons and abilities. The killstreak system from previous games has been refined into a points-based system that gives players more balance. Instead of getting points for each kill, players can choose Assault, Support, and Specialist modes that give them different rewards. Assault is the traditional killstreak system, where you get new abilities like missile strikes for more kills in a single life. Support counts your kills and other actions between lives, constantly moving up and giving you support-based abilities like UAVs. Specialist gives you more perks for your kills, instead of new weapons it effectively turns you into a super-soldier who's faster, stronger, or able to take more damage.
The multiplayer options offer a benefit over Battlefield 3, which only has six cooperative missions compared to over a dozen in Modern Warfare 3. Competitive play is pretty evenly matched, with leveling in Modern Warfare 3 universal and steadily unlocking weapons and skills and leveling in Battlefield 3 and rewarding your class as you play it. The big difference between the two games is that Battlefield 3 has vehicles, which can change the nature of a game dramatically. To go to extremes, Modern Warfare 3 is more suitable for Counter-Strike fans, while Battlefield 3 is more suitable for Halo fans, but the distinction between the two modern combat games are far smaller than the rift between the two classic shooters.
Modern Warfare 3 is a solid follow-up to Modern Warfare 2 and Black Ops, and if you loved those games you'll love this one. It doesn't change anything big, and the single-player campaign wraps up a story with most of the exposition lodged in previous games, but it offers a great multiplayer experience and a solid, if predictable and occasionally frustrating, single-player experience.