I _also_ switched from web.py to Pylons. One of the biggest reasons was a horrible security gaffe that Aaron made -- he recommended doing sessions by pickling the data and directly storing it in a cookie, which is a perfect candidate for exploitation. If he could recommend that on the web.py mailing list, I don't want to use any other software that he could have made security mistakes in.

Aside from that, Pylons is just a better design. Routes, Mako, and SQLAlchemy are an incredibly powerful toolset, without being overcomplicated.