Having quite a lot of experience with clients wanting to build next social network, yelp, online store using word press, I wrote article "Why Wordpress is Bad" - http://www.yepcorp.com/blogs/pavel-karoukin/why-wordpress-ba... a while ago.

tl;dr; Wordpress is great for a blog. I think they put a lot of effort to make UI perfect for single user blog. And it is easy to install. But what happens next - people try to do something with wordpress it not supposed to do. And that's where problems start comming.