To be brief, that Mr. Bingley was agreeable in all ways (nice, easygoing, good-looking), while Mr. Darcy (though fine and good-looking) was disagreeable, in large part because he was so proud.
There are more details, of course. See Chapter III for specifics:
"Mr. Bingley was good-looking and gentleman-like; he had a pleasant countenance, and easy, unaffected manners. His sisters were fine women, with an air of decided fashion. His brother-in-law, Mr. Hurst, merely looked the gentleman; but his friend, Mr. Darcy soon drew the attention of the room by his fine, tall person, handsome features, noble mien, and the report, which was in general circulation within five minutes after his entrance, of his having ten thousand a year. The gentlemen pronounced him to be a fine figure of a man, the ladies declared he was much handsomer than Mr. Bingley, and he was looked at with great admiration for about half the evening, till his manners gave a disgust which turned the tide of his popularity; for he was discovered to be proud, to be above his company, and above being pleased; and not all his large estate in Derbyshire could then save him from having a most forbidding, disagreeable countenance, and being unworthy to be compared with his friend."