I was dissappointed that the writers didn't have Dean throw himself in front of Sam.
I think you underestimate how fast bullets travel. Unless she was a football field away, the bullet left the gun and entered flesh before anybody there could have reacted. Dean was standing in the way (more or less) in the first place; what more could you ask of the man, except that he turn into The Flash and be able to move faster than human perception?
Also, the point was, Sam was still unlucky (and Bela hadn't touched the thing yet); so if anything had ricocheted, it still would have hit him somehow. And done as it was -- that is, firing at Sam rather than at Dean -- it made clear that everybody knows what each Winchester's weakness is. She didn't have to threaten Dean; she knew to threaten the thing he values more than himself.
The part I liked about that scene was Dean's indignation at the very idea that a perfectly sane, unpossessed human being would shoot at him for mere personal gain. One expects him to be angry, but the fact he was shocked charmed me. So naive.