Emma isn't my favorite Austen novel, nor my least favorite; it ranks somewhere in the ever-shifting middle, below Pride & Prejudice and Mansfield Park, and possibly below Sense & Sensibility. However, all of Austen's novels are enjoyable, each for different reasons - Pride & Prejudice for its vivacity and wit; Mansfield Park for the sweetness of its heroine and the story's mellow tone; Sense & Sensibility for the drama, theme, and contrasting characters of Elinor and Marianne; Persuasion for its touch of melancholy; Northanger Abbey for its sprightly, somewhat off-the-wall humor; and Emma for its own hilarity.
There are plenty of other things about Emma that make it enjoyable, such as Mr. Knightley's (pardon the pun) chivalrous character, the twist Austen gives to the romances, and the sheer number of errors that Emma Woodhouse brings about in her attempt to be a matchmaker for her friends. But one of the most interesting points about the novel, I found, was that Jane Austen managed to have such a bratty, spoiled character...and yet make her so likable as well. Surprisingly few people dislike the book, whereas one would think that Emma's inconsiderate nature and harebrained schemes would turn people off. Some modern authors have attempted to have such characters, either with the thought of redeeming them in the end, or merely with the hope of crafting something different from the usual sympathetic character; but generally speaking, these efforts fall flat where Austen's Emma Woodhouse did not.
Several reasons can be found for why Jane Austen succeeded where others cannot, the first and most broad-brush of which is that she was a classic writer and managed to get away with things that the little people cannot. However, not only is this a very depressing and irksome reason, but even if a book is a classic, readers are unlikely to enjoy a bratty main character unless the author does a remarkable job in pulling it off. (An example of this for Austen would be that, classic or no, most Austen fans put Mansfield Park at the bottom of their list because they feel that Fanny Price is a doormat.)
A better reason why readers like Emma Woodhouse is that Jane Austen did not make her wholly snobbish and selfish. Emma is shown as a generally kind and goodhearted young woman whose virtues are sometimes hidden under the fact that she is spoiled and that she has a somewhat wild streak of fun in her nature which often leads her to act before she thinks. Austen does not merely show her in her faults, such as ridiculing the interminable Miss Bates; she also shows Emma's gentler side as she regrets her harsh words, when she watches over her father, and in her affection for Mr. Knightley. An author simply cannot have a character with irritating faults without making amends in other places, or they will alienate readers.
Another, and perhaps greater, reason for Emma's likability is the way Austen contrasts her actions with Mr. Knightley's and has him play the role of guardian and corrector, rebuking Emma and showing her the errors of her ways. These reprimands are gentled by the revelation that Mr. Knightley is and has been in love with Emma, and their fruitfulness throughout reveals Emma's growing character. On the other hand, a last minute change of heart on the part of a character who has been annoying for the entirety of the novel does not work, as it is unbelievable and leaves no room for growth or time for the character to redeem himself.