My verdict: Start with Django; use Flask once you hit 10M+ users.
• Django -- Sensible defaults. A plugin for most things you may need. Fairly high learning curve, but worth it. I've heard people at big startups say that they've spent weeks building things with Flask that were either built-in, or came as a pluggable app for Django. Pinterest still uses Django for their middleware, but switched over to Flask for their API [1].
• Flask -- Low learning curve. Very lightweight. The go-to ORM is SQLAlchemy, which is more fully featured, but has a higher learning curve, and is generally less-liked than the Django ORM. Great for hackathons.
• Pyramid -- Extremely configurable. You get to (have to) make a lot of low-level decisions. Serious lack of sensible defaults. Strong community. The IRC channel is helpful, yet time is spent discussing why Pyramid isn't more popular (lack of sensible defaults).
[1] http://www.quora.com/Pinterest/Would-Pinterest-consider-Flas...