I agree that Elizabeth is not like Jane, whose beauty would catch people's eye at first glance, and you're probably right that her character and quick wit would not necessarily appeal to the "average" man of her time, but I think she's the kind of young woman who would find one or two men of intelligence and good taste ;-) to appreciate her at any more discriminating gathering she attended. I think that Darcy knows better than anybody that Colonel Fitzwilliam is such a man (despite his rather expensive tastes), and the Colonel's appreciation of Elizabeth would make Darcy appreciate her more, too (or at least confirm that he has good reason to be attracted to her).