Well FIRST of all she's just FREAKING adorable.
Second, I feel that she's one of the most unique and dynamic characters. Think about it. She was introduced as a villain, but not as some freakish monster that was wreaking havoc on all of ponykind to satisfy their hunger for power, but instead as a seemingly normal unicorn pony.
She was a very strong and confident opponent to Twilight, always managing to stay one step ahead of her.
After she was finally reformed, she struggled to find her place in this new life of hers. She had troubles gaining the full trust of the Mane Six, she struggled making new friends, and she struggled dealing with her past. Something that is very relatable to most of us here, I'm sure.
Also she's very snarky, sarcastic, and giggly while also being somewhat shy and timid, especially around new people.