Velma is a superhero to me because she's a woman making it on the strength of her intellect. She always has the necessary background knowledge to spot the clue that is out of place. She's brave and stalwart, willing to walk into the heart of the volcano or into the depths of the swamp.
She even has a weakness: blindness (in the form of a tendency to lose her glasses).
I've loved Velma since I was a child watching the original Scooby Doo, Where are You? series, and I've enjoyed the interpretations since in other cartoons and the movies. Especially in more recent programming, both Velma and Daphne get a rewrite and are no longer just comic relief and victim, respectively.
Velma is so hipster that she was cool with her own geek-ness before the world began to realize that smart is sexy. I still wouldn't mind growing up to be Velma.
Don't forget to check out my own superhero stories. Change of Life, book two of my Menopausal Superheroes series just came out a few days ago!