I haven't read the book, so I don't know how much is missing, but aside from her being socially clueless, she really doesn't seem like a bad person. Emma was treating her condescendingly from the get go, talking and joking about her behind her back while being falsely sweet to her in her presence. Mrs. Elton never misrepresented her feelings.
What fun is being cool if you can't wear a sombrero?
http://momismean.deviantart.com/
Emma only began being rude about Mrs Elton when they'd met and spent some time together, which isn't made clear in the adaptation. Mrs Elton is arrogant (assuming she's going to be the leader of society in this village of country bumpkins), Snobbish (bragging about her rich relations and insulting about Mrs Weston), overfamiliar and vulgar (using Jane's first name, calling Knightly 'Knightly' and her husband Mr E) pushy (towards Jane Fairfax, Knightley and Emma) and really rude, spiteful and unpleasant to poor Harriet. There are also hints in the book that she has a rotten temper.
Emma is bossy, high handed and snobbish (at the start of the book) but is also generous and good hearted. She has everyone's best interests at heart, even thought she's often wrong. Augusta's all about playing the great society lady and making herself as important as possible, while hiding under a veneer of false modesty. Not a redeeming feature, a horrible creature!
I lost my jobshare
What? Why?... Not the Phantom Menace?
I always thought Emma already felt some resentment against Mrs. Elton before she even met her, because she'd hoped for Harriet to marry Mr. Elton. Kind of like disliking Mrs. Elton was a way of being loyal to her best friend.
Then after meeting Mrs. Elton, Emma saw how arrogant, rude, and presumptuous she was - being so familiar with Mr. Knightly, and behaving as if she needed to "help better" their society. Those behaviors were apparently extremely rude in their time. I felt that is what turned Emma's initial loyalty-resentment into genuine dislike.