Similar to what @wbmnfktr and @poljpocket said before, I like PW so much because it is an API with a backend, the api is well documented (!), it is so easy to install, easy to update, easy to understand, easy to extend and customize, it can be installed anywhere, it is so direct, I can program simple php templates to output what I want the way I want. I could build simple websites and more complex web applications on my own without the need of experts of this and that technique or paradigm. That's cool!
Please keep it simple, keep it that way and don't overload the core with too many features... ?