Tornado is intriguing, but the main thing I don't like about it is how it reinvents the wheel with a lot of its components. For example:

- They created a lot of Web-stuff parsing code themselves, when Werkzeug provides a tested and thorough implementation of a lot of that. (Especially routing, regex-based routing just looks horribly kludgy once you have used werkzeug.routing.)

- They created their own template language when they could have used Jinja. (In fact, tornado.template is basically a half-as-powerful copy of Jinja.)

- They created their own database access layer when they could have used SQLAlchemy instead.

Sometimes I think people take the concept of "minimal dependencies" way too far.