One of the things that is bothering Anne about Mr. Elliot is that she finds him

"rational, discreet, polished, but he was not open. There was never any burst of feeling, any warmth of indignation or delight, at the evil or good of others."

To her, this is imperfection because:

She prized the frank, the open-hearted, the eager character beyond all others. Warmth and enthusiasm did captivate her still. She felt that she could so much more depend upon the sincerity of those who sometimes looked or said a careless or a hasty thing, than of those whose presence of mind never varied, whose tongue never slipped.