I've not seen SALT. I think the reason it's so hard to get it right is that women aren't particularly adept at acting in that role. That's not so much a sexist thing, but a cultural one. Until, what... the 80s with ALIEN and THE TERMINATOR, women were never the leads in any type of action story ever. As such there is not much of a cultural or historical element in the world for women to draw on in these roles. Women until recently could not be cops and soldiers and so on. Women as a physical power is only very recent, and it's been gradual. Action women are primarily the protagonist of horror movies, in which it's physical but they're a victim. The proper action heroine is really still in its fledgling state, so it's harder for women to pull the roll off. It's kind of like green-screen acting. The totally immersive green-screen in which there is just no props at all, just one guy in a costume, is really rather new. The only place it's been done before is really theatre which is very abstract in its sets when it calls for epic scale (such as Shakespeare) as opposed to more mundane one-room-in-a-contemporary-house type of affair (like Ibsen's A DOLL'S HOUSE). But on the whole, it's so new most actors can't pull it off. It's not that it's impossible, it's just too new. Same for women as action heroines. And it's also true on the other side of the camera; directors aren't sure how to direct them. There are exceptions of course; Tracey Scoggins was very good in BABYLON 5, very tough and strong, as was Charlize Theron in IN THE VALLEY OF ELAH; Karen Allen as Marion in RAIDERS OF THE LOST ARK, Carrie Ann Moss as Trinity in THE MATRIX. Angelina Jolie seems to be able to do it too. But the directors often find it hard to not direct the movie as though it were a strip show; consider Johannson in IRON MAN 2. She was fine as the Widow but the movie treated her like a strip tease.
I think we need a Toph movie.