Flask is great for beginners because it's well documented and easy to understand. You can become familiar with Flask in a weekend -- start with the Quickstart and then go through the tutorial (http://flask.pocoo.org/docs/).
You won't have to spend much time learning it or fighting with it -- you won't find yourself asking, "Will I be able to do what I want in the framework without hacking it?" Flask let's you program in Python rather than writing to the framework like you typically have to in larger, opinionated framework's like Django and Rails.
Ironically, this also makes Flask an ideal choice for advanced Python programmers because it gives you flexibility rather than always wondering "will the framework allow me to easily do...?"
BTW uwsgi is a production app server. For example nginx has a built-in uwsgi connector and you use uwsgi to serve Flask apps (see http://flask.pocoo.org/docs/deploying/), but it's not a framework like Django/Pyramid/Flask.
Flask to teach you about web services in general, and then move on to Django once you have a decent grasp of things.
Flask is still excellent for small to medium, toy sized projects. Weekend hacks thrown together as a demo, for example. Beyond that I've found the lack of structure and third-party applications in Flask to be a hinderance for more mature sites. I'd say that for any given feature I need on a site that isn't extremely specific to the purpose of it, 90% of the time I can find it on django-packages and have it dropped-in and integrated within 10-15 minutes.
I'd venture to guess that for every hour I spend getting something to work that doesn't fall nicely into Django's structure, I save a hundred or so from not having to re-write a mature component that the community has solved ten times over.
I use http://cherrypy.org/ (behind a nginx revproxy) to my satisfaction. The data is stored either in http://www.mongodb-is-web-scale.com/ , or in MySQL. Disclaimer: these websites are not designed for 50.000 hits per second, but during benchmarking I get consistent times like 4 msec per page, and I'm confident that nginx can handle many slow clients simultaneously.
