OK, well you lot have argued the hell out of this question, so I can't see any of my points being that good, but I will give my opinion, seeing as this is what it's for.
I think that every character on the show has good points and bad points. However some people's bad points overshadow the good, and I believe that that is what happened with Kennedy's character. That, and the fact that we only had one season to get to know her in. Maybe, after a couple of seasons we would have grown to like her for her sassy, confident personality. But because we only had one, I found those traits startling and irritating. She was good for helping with the Scoobies, and she gave Willow support in her healing after Tara's loss and her magic overload, but apart from that, I just found her bossy.
Also, I don't think the chemistry between them went on long enough, in a few episodes they were together, whereas Oz and Tara were carried on for longer before Willow went out with them.
And I just think that Tara's character was subtler.