Tornado is batteries included. Flask uses Werkzeug and Jinja2 and encourages packaging extensions for localization, form processing, ORM's etc.. Flask, Werkzeug and Jinja2 are all Pocoo projects that have a good community around them and excellent documentation. Technically I would not like to argue the superiority of either, although Flask seems to fit my tastes much better.

Flask's use and deployment of "magic" is pretty spot on. For example by making the request object omnipresent (that is flask.request is always bound and has the right context). That way it is not passed around the MVC stack but just imported and introspected when needed.

Flask is probably the "healthiest" project I've encountered.