The main thing I wish I had known is that the ORM is rather limited (or, at any rate, it was too limited for my app), and while you can use raw SQL, it won't let you get model objects out of your own queries, which makes it impossible to integrate with the rest of your app that uses the ORM [1]. The first thing I'd do if I had to use Django again would be to eschew its ORM and use SQLAlchemy instead.

[1] This was a few versions of Django ago; if they fixed this since then, I welcome corrections.