Salt has most of the ingredients for a tasty, Bourne-style espionage casserole. It just doesn't have them in the right amounts. You need action sequences, of course, and a butt-kicking hero, and a certain degree of uncertainty about whether the good guys are all completely trustworthy. A nefarious plan to assassinate, oh, say, the president adds flavor. For a nice retro feel, it doesn't hurt to bring back a favorite villain from yesteryear, like the Russians.
All of the above is to be found in Salt. What it lacks is motivation. Maybe that doesn't seem very important, but you sure notice when it's missing.
Our heroine, Evelyn Salt (Angelina Jolie), is a top CIA spy who's accused of being a double agent. If this is true, it would be a fairly serious blemish on her employee evaluation. She says it isn't true, but that's what you'd expect a double agent to say. The film is about her efforts to prove her loyalties, protect her civilian husband (August Diehl), and prevent the real bad guys from carrying out their plot.
The problem -- and I'm trying to avoid spoilers here -- is that we're never given any sign of WHY she does the things she does. In a story full of double- and triple-crossers, she seems to have been chosen to fill her particular role arbitrarily. The purpose she serves in the story could just as easily have been served by one of several other characters.