So let's get the obvious out of the way, okay?
Gone With the Wind is racist as fuck. No, seriously, it has that pernicious "let us tell history in a completely false way - look weren't slaves happier being slaves? hooray for the Confederacy!" thing to it that, pardon my language, can fuck right off.
But setting that aside, Scarlett O'Hara is an amazing character. She's not nice. She's never even close to nice. She's spoiled, self centred, egotistical, vain, greedy, has little to no morals when it comes to getting her way even if it means ruining people's lives, runs after the wrong man for the entire book, and doesn't realise what an amazing friend Melanie is.
But I have this to say for her: she's an incredibly strong character. Margaret Mitchell didn't pull any punches when she wrote her - didn't try to make us like her, didn't attempt to excuse her faults.
So, reasons why she's awesome:
She never wanted to get married, or have children and was surprisingly outspoken about it. I think marriage is great if you want to get married, but Scarlett didn't and considering the time period (both of when the story was supposed to be happening and the time the book was published) it was pretty brave.
She's unrepentantly self-absorbed, vain and she doesn't even attempt to understand other people. War? Death? Illness? Love? If it's not about her she won't care. Okay, this isn't a plus for the character - this is a plus for the author - because she managed to make the reader root for Scarlett despite this.
In spite of being one of the most spoiled characters ever, (or perhaps because of this), she has tremendous strength and courage. She believes she will get her way, and she does get her way even if she has to do it all herself and step on everyone else to do it.
She doesn't suffer fools gladly, even her precious Ashley! She often catches herself thinking how weak he is - when she kept white convicts working for her and he went to talk to her about how shameful that was, she fired right back that he'd had no problems keeping black slaves, so what gives?
She realises her mistakes and grows with them. You can't say she becomes a better person - that's open to interpretation, but personally I don't believe she does. I believe she'll keep doing whatever she needs to do to get what she wants. She just finally realised she wanted the wrong things. That's growth, and realistic growth at that because, in my opinion, people don't change who they really are.
But most of all, she's strong. Whatever life throws at her she faces it head-on, uncaring of what others may think, defying every odd. And she always comes out on top. She's not a nice person, but you can't help but admire her, anyway.