Subtle direction (the light fixture breaking with the off camera gunshot,) great script (fast-talking salespeople conning each other) and excellent pacing (plenty of big events occur off stage but aren't belabored by tedious exposition.)
Sure, Stewart was kind of an a-hole by toying with her for so long, but this is overall a tightly-paced and funny and heartfelt movie.
Hmm it's strange how people have such different opinions of the same film, each one of value to the owner.
I spent most of the film thinking she was a bitch, who should have been fired for her disrespect of those put in authority over her. On discovering why she had acted so, it turns out she wasn't just a bitch but a stupid, manipulative one as well.
Stewart let her off easy.
She did indeed make some of the nastier remarks and had an attitude, but you really thought his personal insult about her being an old maid was nice? And why did he have to toy with her for so long, even breaking her heart? Once she found out about that, she should've been a bit pissed at him.
Manipulative is an exaggeration, she wanted to play a little hard-to-get, but he reacted badly to it and they got stuck in a vicious circle.