So some of you might be wondering--of all the thousands of characters in the Star Wars universe... why Mara? Well, it's quite simple really.
She kicks ass. She kicks it well.
Not to mention that she's probably one of the coolest overall characters that's ever come to Star Wars. This I find simply amazing because she's probably as close to an original character as a canon character can get. Tim Zahn, who created her, probably never imagined that his brainchild would go so far and become such a staple character of the Expanded Universe, but when you take a look at how completely awesome she is, you know why she has.
With a past complex and endearing (because, hey, it's one of the best chances the authors have to explore the inner workings of Palpatine's reign) and emotions that are real and understandable, people can't help but fall in love with the character. And hey, Luke sure did ;D. You've gotta be pretty damn cool to be written into the plot as Luke Skywalker's wife, right?
RIGHT.