1: Trying to cram every badass characteristic into one design.
If you're character has sunglasses, a long coat, a sword, magical powers, vampirism, demon wings, tattoos, long anime hair, eyes that are two different colors, and dual pistols, then you may have just created the most obnoxiously blatant anti-hero there ever was.
When designing a unique character, you have to commit to one design, instead of combing all of your favorites into one confused abomination of a badass. There is nothing worse than excessive accessories and something so busy looking that it resembles a Second Life avatar wearing all the custom content you could possible fit.
2: An original character of you.
Most people want to create something that represents them, but an accurate representation of yourself just isn't interesting. A shy computer consultant with glasses that would never let anything happen to their loved ones is not a good character design.
If the character is based on you, you have ...