My vote goes to Django as well. I started hacking a reddit clone last year and there are many apps to get you started (comments, votes, private messaging etc).

My vote goes to Django as well. I started hacking a reddit clone last year and there are many apps to get you started (comments, votes, private messaging etc). You may find that the apps don't exactly fit your needs and that's when you start learning even more, by looking their code. One of the main problems I had was user permissions for each post and category (subreddit) and what that means for caching and memory management. At any point you will need to know if a user voted on a certain post and whether or not they are allowed to edit or delete it based on what permissions the user has and in which category it was posted. Threaded comments are also a bit of a pain when it comes to managing memory - I've read somewhere that Disqus uses a built-in PostreSQL function to build the tree but I haven't tried it.