What makes Wordpress unique I think is that a lot of great web designers use it for their own portfolios and blogs. It is not mainly used by coding geeks and focussing more on technicalities that look and feel. Having loads of designers on board that further find it extremely easy to then implement complex and detailed designs in Wordpress have put Wordpress in a position where the many themes available have a unbelievable high quality and most of them are free. Biggest challenge is to find the right theme in the endless supply