There is nothing particularly wrong with this trailer per-se. It's neat to see Tom Hardy in a comedy, Witherspoon looks stunning, and the general plot (two CIA agents clash when they realize they are dating the same girl) is fitfully amusing. But here's the problem. In the first 66 seconds, we meet the two spies (Hardy and Chris Pine) and the young woman they are both dating (Reese Witherspoon). We establish the conflict, toss off a few jokes and, with the opening action montage, establish that this will be a pretty big-scale action comedy. So, after about 1:06, we have all we need. Flash the title, maybe give a button gag, and then fade to black. The remaining 85 seconds merely exist to spoil gags from the film. And since we've established that this is a pretty expensive movie, it's pretty much guaranteed that the film will only have maybe half-a-dozen or so major 'give-and-take' moments between the two would-be suitors. The trailer sees fit to spoil three of them, plus a major action sequence between the two male leads. So basically all that is left for the movie to do is reveal whatever gags weren't in the trailer and reveal the finale*. How wonderful would it have been for 20th Century Fox to merely leave the majority of the movie to our imagination, to merely cut to black at the 1:06 mark with the satisfaction of perfectly setting up the movie without actually giving anything away past the first twenty-minutes or so. Alas... Anyway, McG's This Means War comes out February 17th, 2012 (better to capitalize on Valentine's Day) and as always, we'll see.
*SPOILER GUESS --- I'm guessing that either Witherspoon gets kidnapped by the bad guys in the third act or (especially since she gets first billing) she turns out to be a spy using them for intel.