ForeignKeys need to have the on_delete Attribute set (e.g., to models.CASCADE for a cascading delete)
This also affects existing migrations. If you have migrations that you created with Django 1.8, you will run into errors (as they do not have that attribute set in the migration).
Also see this Ticket: https://code.djangoproject.com/ticket/28677
Apps should specify the "app_name" attribute in their urls.py
If you don't do this, you might run into an error when you include that urls.py in another urls.py
Also see this Ticket: https://code.djangoproject.com/ticket/28691
SessionAuthenticationMiddleware is no longer available
If you have SessionAuthenticationMiddleware MIDDLEWARE listed (you most likely do if you are upgrading from an older Django Version), you will have to remove it from your middleware list (or tuple).
user.is_authenticated() and user.is_anonymous() are no longer available as functions
They are now properties and have to be called without the function parantheses!