Casting Iben Hjejle was a major mistake.

I can understand why you feel that way. He performance wasn't terrible but I do think she was miscast. I still love this film so much.
I think the point of casting a little know actress who is pretty but not a super model was to emphasize the films theme of maturity. Laura is a very good girlfriend but Rob wants the fantasy girlfriend which Laura isn't.
The Laura character is better developed in the novel. If you haven't read it I highly recommend it.

Upon subsequent viewings I agree. Her performance was very wooden and dull.

Don't listen to these bums, Iben. You were great.

It was a thankless character to play. Laura's the mature one, ready to settle down. She already gone through her own midlife crisis by the time the movie opens -- it's why she broke up with Rob.

As a result for most of the movie she just sits around and waits for Rob to grow up. She makes interesting faces every now and then, but it's a pretty one-dimensional part.