Django is great if you're making a standard cms type website. News feed, blog, etc. There's a bit of a learning curve but it gives you a lot of the scaffolding out of the box (and there are loads of examples to learn from). It has the biggest community (I guess?) so you'll always be able to find answers / libraries / sample code.
Use Flask if you really want to get a website up and running with half a dozen lines of code. You'll have to build everything else yourself; admin, authentication, etc etc - though there are good extensions for a lot of things too.
Thinking about it - if you don't know either of them, spend a day with Flask, by the end of it you should have a fairly good idea of what you get with it. Then run through the Django tutorials. It'll take longer but you'll be impressed with how quickly you can get something fairly complete up and running.
For me, I lean towards Flask (heavily). When you think about what Django does - it's basically a web container with a db/model layer and sensible scaffolding for the views (front and admin). I prefer to use SQLAlchemy (if I'm working with a relational db). In terms of the admin I think that the days of writing admin grid and edit screens that post back and forth to the server are numbered (I use REST and Angular now) - so that's not a bit of Django I use either. By the time you've ripped all that out you may as well save yourself all the config and just use Flask. Having said that - LEARN THEM BOTH. At least just a little bit so you can see for yourself where each one shines.
These days I approach it completely differently. My last two projects have involved a fair amount of complex business logic (and processing work). I've written the libraries I needed first as standalone packages, only introducing Flask as a final step. The web wrapper is just there to provide a web interface (mostly an api) access the db layer and glue it all to my libraries. I'd recommend this approach as it stops you from thinking along the lines of "ok, I'll need to create 3 Django apps, here are my models for each, should I put this code in the models.py or the views.py?" Instead you concentrate on making the real python code to do the heavy lifting, and you architect that sensibly without having to worry about how the web framework would normally prescribe it to be done. You'll end up with far more portable code in the long run.
The creator and lead developer, Massimo Di Pierro, is incredibly active and always willing to lend a hand with whatever problem you have. I've seen him around reddit (not sure if he's on HN) and even when he's met with the inevitable haters, he's always been friendly and cordial.
Just like you've said, web2py was originally made to teach people how to build websites, which really shows in its architecture. It forgoes traditional Python conventions ("explicit is better than implicit" being a major one) in order to be more beginner-friendly. Some of the more experienced developers find this annoying, but if you're coming from PHP or a fresh background, it can be much easier to get started. Just take a look at the overview on Wikipedia[0]. It gives a good outline.
This is coming from someone who doesn't even use web2py. I prefer Flask for personal projects and Django for work/group things, but I would whole-heartedly recommend web2py. Run through the book[1] and, if you don't like it, give the Django tutorial[2] a shot. If that doesn't mesh with you, there's Flask[3], Pyramid[4], TurboGears[5], and CherryPy[6]. (The last two I would not recommend for beginners, but they're still good!)
[0]: http://en.wikipedia.org/wiki/Web2py#Overview
[2]: https://docs.djangoproject.com/en/1.5/intro/tutorial01/
[3]: http://flask.pocoo.org/docs/
[4]: http://docs.pylonsproject.org/projects/pyramid/en/1.4-branch...
My feeling too. Are there any projects which are starting to create the boilerplate for this? I'm new to Angular, so looking for where to get started...
It should be possible to build a completely generic frontend that just has a grid view and an edit view that you could wire to any REST backend. Then you could use flask-restful or Django Rest Framework or whatever else you wanted.
If you were going to go that route there's an angular directive for doing grids that would get you half way there: http://angular-ui.github.io/ng-grid/#/examples
Were I still building CMSs that's the approach I would be looking into.
However, for large web projects, Django beats it hands down. Doing similar work with Flask would require so much wheel-reinvention that it'd be hard to justify your time or your client's money when it's all built quite well already.
Of course it's not perfect, and it's not for everything. But it's a great general-purpose, medium-performance web framework. It becomes a pain on very high-traffic sites (perhaps more so than JVM-based solutions and on par with scaling Ruby frameworks), but if you use it in the right place and/or team up with systems people that know their stuff, it's a powerful tool for quickly building complex websites in a maintainable and intuitive way.
IronPython?
Also Heroku has been mostly a godsend, as I haven't had to spend nearly any time on sysadmin stuff. One thing that caught me early on, not sure if it's a bug or what, but in Heroku, if you use git URLs for some of your packages in pip requirements (which is useful if you need to customize a package or more frequently fix a bug and can't wait for it to be pushed to pypi), Heroku's package cache has trouble knowing when to update unless you a) change the package version in its setup.py (may or may not be a good idea) or b) change the Python version in runtime.txt (kind of a pain as generally that means two deploys for an update, bump down one minor, then back up).
They just wanted a simple web form, where the sales people could add notes throughout the day and then automatically email those notes to the owner at the end of the day.
I first tried Django and was quickly overwhelmed.. I switched to Flask and with the help of flask-login, flask-wtforms, and flask-mail, it only took me a long weekend to get it up and running.
Just a personal anecdote, but Flask was much easier for me.
This doesn't make it better, but it also doesn't make it worse.
If it's relatively simple and you don't really want to deal with user registration/databases I'd say definitely go with Flask. Django does a pretty good job of those two but until it "clicks" you end up being confused.
"However Flask is just not designed for large applications or asynchronous servers. Flask wants to make it quick and easy to write a traditional web application."[1]
Flask is excellent when you just don't need a bunch of the stuff in Django, and don't need the batteries to be included. It's not so much that Flask is suitable to high-complexity apps, but more that it's suited to highly custom web apps. Building a music sharing app based on jQuery mobile and backed by Mongo? You're better off with Flask. But at scale you're going to end up doing something custom.
Django is highly suitable if you need to manage users, have a variety of entities that lend themselves to the admin dashboard (more broadly, that the admin/public app metaphor even makes sense in the first place), and find a plug-and-play package system compelling.
They are both great frameworks, but "complexity" (advanced/simple) is the wrong axis along which to evaluate them.
The documented limitation you reference is based on the WSGI server being used. "If your server uses some kind of concurrency that is not based on threads or greenlets, Flask will no longer be able to support these global proxies."[0] As nginx+gunicorn is becoming more popular, this isn't really an issue at all.
[0]http://flask.pocoo.org/docs/becomingbig/#scale-like-a-pro
Armin has said before that he needs to update/clarify that statement -- as dvanduzer said, it's not really relevant anymore.
Django is definitely more in the range of "we want you to write" this way but I've found it to be at least a bit more customizable than RoR. Flask is on the other extreme which lets you do whatever you want.. but you're the one who has to do it.