My thoughts on the gulf between Meryton (the merrytown) and Darcy are that it is Darcy who sets himself apart from the community. Everyone was prepared to like him, even better than they did Bingley, on account of his fortune, his grandeur and his fine figure (ahmmm!). And thinking about this, perhaps it is partly that which makes him recoil from their civility. From chapter three (this is one of my favorite passages):

"...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."

When I read the phrase "to be above his company", I can picture him being there, enduring the presence of people whom he considered beneath him, disdaining the hospitality of which he was the beneficiary. This attitude would not make anyone any friends anywhere. Their dislike is well deserved, imo. I know someone who behaved like this at a party to which I had taken her, and my embarrassement and mortification knew no limits.

I think JA lets us see exactly how Darcy behaved by describing Bingley's demeanor in the same situation:

"Mr. Bingley had soon made himself acquainted with all the principal people in the room; he was lively and unreserved, danced every dance, was angry that the ball closed so early, and talked of giving one himself at Netherfield. Such amiable qualities speak for themselves. What a contrast between him and his friend! Mr. Darcy danced only once with Mrs. Hurst and once with Miss Bingley, declined being introduced to any other lady, and spent the rest of the evening in walking about the room, speaking occasionally to one of his own party."

Mrs. Bennet says that Darcy was high and conceited and that "he walked here, and he walked there". I always picture him walking about like a peacock in this scene (CF did a great job). He really behaved rudely and not at all like a gentleman should. JA calls him haughty, reserved and fastidious, and I would add, petty. He judges people at first glance, not based on their character, but on superficial traits. Rather than one particular event, I think it was a general dislike of his behavior among the town, though Mrs. Bennet has further reasons to resent him.

JA also says that his manners were well bred, but I don't think he displayed any evidence of that in the assembly.