Bland is what comes to mind when i think of Portman. I haven't seen BS yet so that may change but nothing i have seen her in so far makes me believe she's brilliant.
I don't know, i personally don't think she has that aura, that gravitas, most of the time she's just serviceable as if she can be easily replaced with any other cute actress in any role she has done without a drop in quality.
I did think she was effective in Leon and Closer ( most people do look good playing off of Reno or Owen ) but once again, i feel you could have replaced her with anyone else and it wouldn't have made a difference.
I don't think she elevates the material, she gets the job done as you would expect any competent actor .