In order to be a movie star, I think you should have at least one (although preferably more) of the following qualities:
1. Can act amazingly well (i.e. Paul Giamatti).
2. Is ridiculously good-looking (i.e. Halle Berry).
3. For some reason, is just really likable (i.e. Sandra Bullock).
The very best movie stars have all three qualities, for example, I would claim that people like Helen Mirren and George Clooney have all three, whereas people like Halle Berry or Angelina Jolie really only fulfill the second criterion. But their beauty is so ridiculously extreme, that it's okay. Even when their movies totally suck (as they so often do), you can always just space out and marvel at their inhumanly impossible good looks.
Category 3 is kind of a wild card. There are always a few movie stars who have a certain kind of presence and charisma that makes you like them, even when they can't really act and aren't particularly good-looking.
Which brings me to Kristin Stewart. She does not have this quality. I do not like her. To me, she has nothing going for her--no acting ability, very average looks, and inherent dislikability. And yet, she is poised to become a huge movie star, just because of those damned teenagers with their stupid vampire movies. Nooooooooo!
The above clip of Kristin "acting" with William Hurt is so incredibly corny and bad that it almost seems like a parody. Sadly, it is not. It's from an actual upcoming independent film called The Yellow Hankerchief. In addition to her usual terrible acting, Kristin is also attempting a Southern accent here.