Ask HN: What did you learn in building your scalable web application?

I have built couple of client-side API mash-ups. I have only worked as an intern on a big (in terms of users) web application. I am planning to build a Django application on AWS for a large user base. Though I can build a quick prototype on Django, I just have no experience to learn from.

What are the common 'gotchas' in building a scalable application in Django on AWS? I intend to learn from industry experiences. Please tell your personal experience and give some pointers to blog posts or books.