I think he was attracted to her first and foremost because she wasn't intimidated by him, and she wasn't coyly trying to "catch" him. She wasn't afraid to speak her mind to him because she wasn't interested in him as a potential partner. It freed her up to be herself and as a result she was more genuine than a lot of women in his sphere who had "set their caps for him," so to speak, (aka, Miss Bingley.) There was no pretention with Elizabeth. She let him see exactly who she was. It made their eventual romance so much more satisfying because it was based on who the two of them really were, and not any kind of pretense or façade that courting often creates and encourages.