I have been doing a lot of research on Tarantino's depiction of gender in his films, and I am wondering what it is about his female characters that is appealing for female fans?

In the simplest terms, it's because he doesn't depict women as submissive, incapable, inadequate, docile, and meek cliches. They are strong willed, enduring, and possess the strength and integrity of any of his male characters. For example, in Kill Bill, when the bride is fighting each of her enemies, you see every battle as a battle between warriors. The Bride fighting Vernita Green is not a ”girl fight”, it's not a catty sex display. It's two assassins fighting to the death with honor and dignity, just as if it were two men.

Well, the person above me already summed it up better than I can.
We admire how he's made women with all kinds of personalities, but none of them are portrayed as weak.