Well, I like the simple approach to web development that web.py offers and the fact that you get to essentially lay out your own directory structure and pretty much create your own framework. But what really attracted me to Pylons is the interactive web based debugger. If you haven't seen it yet you need to give it a try. In "development" mode it will give you a page with the actual traceback and you can interactive inspect the stack, check the value of variables or just run arbitrary python, all over the web. Makes debugging a lot easier.