I also found Kathy's blog very insightful and it's a great tragedy that she stopped writing.

An eye-opener for me was her "Is Twitter TOO good?" post: http://headrush.typepad.com/creating_passionate_users/2007/0...

For me, this opened up how I thought about products, especially in terms of psychology. I can't really remember when I have read that insightful posts from anybody else (and Kathy had a ton of them).