The character of Elizabeth Bennet has a penchant for speaking out her own views of everything, and for forming strong opinions. These opinions, however, she would later on treat as facts. Not only does she believe in these facts, but she also defends them against the opinions of others.
Although most of her views and opinions are valid, there are some that she formulates without much substantiation. The most obvious one is her overall opinion of Mr. Wickham.
Wickham's charms seem to dominate Elizabeth, who hangs on to his every word and believes everything he says. She even believes his sad story of how that "typical" bad, aristocrat Mr. Darcy took everything away from him, leaving him no more options than to "lower" himself to join the military.
Elizabeth's tendency to treat as a fact every one of her opinions leads her to conclude that all aristocrats are bad and they all hurt people because they are all money-hungry and snobby. Hence, in her mind, Mr. Wickham is indeed Darcy's victim and Darcy is "the" prototypical mean aristocrat who hurts other people.
This is an opinion that she continuously drags throughout the story until the moment when she finally realizes (through Darcy's letter) that Wickham is actually a liar, and a man with no backbone, nor dignity. This is even more stinging when Wickham elopes with Elizabeth's younger sister, throwing away their friendship and bringing shame to the family name.
Hence, it is safe to assume that Elizabeth's problem is not that she is opinionated, nor that she is a firm believer in her own thinking processes. The problem with her is that she allows emotion to come in between reasoning sometimes. This is why, in the end, she learns her lesson in the hardest of ways: Through the shameful situation of her own sister.