ORM - you'll write a little more code with Flask + SQLAlchemy to get the same result as using the Django ORM. With Flask, you'll have the flexibility of being able to, say, change a repository implementation to use NoSQL for some set of your models.

Authentication - Django provides UIs that you can skin/replace, and there are Django applications to do OAuth for you. With Flask there are plugins, you have to provide a UI (and UI workflow).

For almost everything Django can do, there is Flask plugin.

You might be able to put together simple sites faster with Django, you will get more flexibility with Flask.