Why was Daisy appalled at Gatsby's party?
Tom and Daisy come from East Egg. East Egg is where the old money lives. It is the place where people who have grown up with money live; therefore, they have a particular attitude toward wealth. I would call it "snooty." Their wealth and association with East Egg grants them a certain amount of status and reputation. They believe that certain behaviors should be adhered to when a person is wealthy.
Daisy is appalled at Gatsby's party, because she looks down on the new money that lives in West Egg. She believes that those people do not behave in the standard way that wealthy people should behave. Daisy sees a lot of drunken debauchery at Gatsby's party, which shows her how low class the new money is.
“Lots of people come who haven’t been invited,” she said suddenly. “That girl hadn’t been invited. They simply force their way in and he’s too polite to object.”
The next quote also shows her feelings:
But the rest offended her — and inarguably, because it wasn’t a gesture but an emotion. She was appalled by West Egg, this unprecedented “place” that Broadway had begotten upon a Long Island fishing village — appalled by its raw vigor that chafed under the old euphemisms and by the too obtrusive fate that herded its inhabitants along a short-cut from nothing to nothing. She saw something awful in the very simplicity she failed to understand.