I think for a web framework to be successful (all other things, like features, being equal) it needs a few essential ingredients:
- plugin ecosystem. Not necessarily at the beginning, but tha architecture should make it easy enough to add plugins (or apps, or extensions or whatever they are called by the framework) that developers feel encouraged to do so.
- high profile sites or projects, or at least a high volume of sites, so developers can answer manager's question, "Well, who uses it ?". Again, no framework will have this at the beginning but it will need this to get big (bit of a chicken and egg situation).
- good documentation, preferably but not exclusively online. Good == well written, up to date, well organized and consistent.
- good marketing: maintainers who can blow their own trumpet in the blogosphere, conferences, books, Twitter etc etc.
Now the code may be crap, and it may not have this feature or that feature, or it may or may not be easy to extend or customize. But the above points are required if it is to stand a chance.
Django, despite the flaws you mention (and I agree with some of them, and could add a few of my own) meets these criteria. It has the sites, the documentation, a healthy ecosystem of apps (though of varying quality), and is marketed by people who work, or used to work, in the newspaper business.
Flask, as an up-and-coming framework, is just a couple months old, so obviously doesn't have the ecosystem or the high profile/high volume yet. But it is well-positioned to take off : the documentation is excellent, and it has a good plugin architecture, with about a dozen or so "official" extensions already. Whether or not it does take off depends on many other factors, but I'd say it's future looks quite promising. It may need some aggressive marketing, or at least one or two high profile sites.
Pylons, OTOH, doesn't look so healthy. Documentation, last I looked, was out of date, poorly organized, and contradictory (AuthKit is recommended in the docs, newbies use AuthKit, get confused by its crapiness, go on IRC, and get told "don't use Authkit" - why is it still in the docs at all ?).
It's not so easy to add plugins, except at the WSGI level, which is often the wrong level of abstraction, plus there is the confusion of "entry points" and how to make it all work with Paster. As a result (and also because of the pisspoor docs) despite being around almost as long as Django, Pylons doesn't have a good plugin ecosystem. It has one high profile site - Reddit - but otherwise for the time it's been around I can't think of many others.