What I like about Flask is that it's got a few things that make it ideal for small projects over something like Rails, Django, or Pylons:
Locality of reference
All my routes and view functions are in one place, no need to track them down between a controller, model, and view folder. Obviously, this doesn't necessarily scale well, given that the main file will be a pain in the ass to edit before long. That being said, we're looking into using Flask and it's Module architecture for a larger project.
Simplicity
Given that it's a 'microframework', Flask doesn't come with the huge file structure and extra utilities that big frameworks do. I like that. It also cuts out the controller; routes are applied directly to the view functions, which manipulate the model. The repoze.bfg framework does this as well, and I think it's a sensible approach, at least for small projects. Again, I'm not sure how this will scale, but it definitely cuts down the learning curve.
Amazing documentation
While Rails has great documentation, I have to say that Flask stands alone in documentation among the Python frameworks I've looked at. Django's pretty close, but also really big for what I was planning on doing. Also, their 'hello world' example on the main page is awesome, because it really does give a flavor of how the thing works in only a few lines of code. None of the other major ones can say that.
Loose structure
Flask seems to be largely hands-off in determining how your code should be structured, again unlike the larger and more well-known frameworks. This also means there's mode code you have to write, but you're not forced to program in someone else's style.
Active community
A really active community has sprung up around Flask. There are plenty of extensions getting written, and people are experimenting with larger projects. It will be interesting to see where they go.
So, while I'm sure I'll end up pulling out one of the existing, bigger frameworks for something, I'm going to work with Flask on my projects, both in my free time and at work. We've started on doing some REST-based peer-to-peer work this week, and it'll be interesting to see how that plays out.
