How do you handle user-submitted image tags? Do you allow rich formatting, and if so, how do you sanitize it? (see http://www.codinghorror.com/blog/2008/08/protecting-your-coo...)
I think that Django uses sha for password hashes. They should use bcrypt, right? Did you turn on XSRF protection (which I think is off by default)? Are cookies secure?
Web security is not as simple as 's.replace("<","<")' (escaping by default).
How do you generate slugs? Could someone put something nasty in a pathname or URL?
Django is not secure. You can secure it, with minimal effort, if you keep things radically simple. But you do need to know what can go wrong, so you don't introduce any "features" that are actually "gaping security holes".
Even if you do everything right, it doesn't mean that Django is magically secure no matter what people use if for (obvious, yes, but this is HN and sometimes failing to point out the obvious can get you downvoted). That's why people are objecting.