No matter how many ways you slice it, the Black Cat is a shallow, two-dimensional rip-off of the Catwoman. Both started out a professional thieves; both have cat-based motifs and identities; both are very skilled in the martial arts and gymnastics; both use the art of seduction to distract their opponents or manipulate allies and enemies alike; both are amoral and motivated by self-interests; and both were in love with the hero but not with his civilian identity. Heck, their first names, Felicia and Selina, even rhyme (okay, thats being picky). Sure there were some differences between them about their backgrounds, and at one time the Black Cat had bad luck powers. However, I often wonder whether or not giving her powers, having her give up her life of crime, or making her act like such an incompetent airhead was because the writers wanted to make her less of a copy-cat, if you pardon the pun....
Personally, I think that the only reasons for her popularity are that she is a lot like Catwoman and that it was pretty clear that Peter was having sex with her; and since Peter was someone the readers were supposed to identify with, then they too could feel like they were having sex with her as well. (Heck, there was even one point in which Mantlo wanted Felicia to be pregnant with Peters illegitimate child, which was fortunately was overruled by then EIC Jim Shooter.) Not that Peter becoming involved with someone who was clearly wrong for him on so many levels is a bad thing in terms of drama. The problem was that she was too much like the Catwoman and all the additions to her character to make seem less so made her shallow. Also, because the writers kept insisting on having Peter and Felicia bump uglies, it got in the way of Peter and MJ ever rekindling their romance properly before he asked her to marry her for the second time. The most egregious example of this was in ASM #289, in which after Peter has a one-night stand with Felicia, and MJ walks in the next morning, he realizes he still loves MJ and finally they admit their true feelings to one another. Then the next issue Peter pops the question? Yeah right.