Well...I'm just gonna take a wild guess here. She is practically one of the four main characters, a member of Team 7. Naturally, one expects more from the show's heroine than other supporting characters.
But she does nothing.
In the first season, she bites a guy. That's it. Other than that, all she talks about is Sasuke and then she cries for whatever reason.
Then she made such huge progress in the beginning of Shippuden, but that did not last very long. Which is a shame, because she was awesome in her fight against Sasori. But then she faded into the background once again.
As for Sasuke, I don't think people are mad at her for loving him. It's how it's portrayed. She had a one-sided crush on a guy who was such a jerk to her, and now she is madly in love with him, even after three years of him being a total nutjob. When she could do something, like helping Kakashi when he briefly fought Sasuke, she cried once again and froze.
I don't hate Sakura, but that is no way to treat the show's heroine. It's not like any other female characters are any better, but there you have it.