I came across this post (posted in March last year) in which Michigan State University PhD student Randy Olson analysed the posts that users submitted to Reddit between its inception in 2005 up to November 2012 in order to retrace the evolution of the community. Olson took a snapshot view of the most active subreddits, ranked by number of posts submitted, in one week in of November 2012. The primary content is (unsurprisingly perhaps) pictures and short videos.
Then he graphed up (see above, or here for a larger version) the 24 most active subreddits, ordered by the time that they first appeared, to show how the volume of content as a proportion of the total posts has changed (N.B. Olson excluded the non-specific /r/reddit.com which disappeared in 2011 for visualization purposes, which explains why it looks like everything on Reddit in 2006 was NSFW, which clearly it wasn't).
Olson breaks down his analysis by year, showing how each year saw changes in focus. It's fascinating to see how Reddit started out dominated by techie-friendly subjects like programming, politics and science but then evolved over time as it matured and the diversity of subject and content type increased.
Olson finishes his analysis by saying that in spite of the growing domination of image and video based content it's clear that there are new subreddits dedicated to just about any topic you can imagine. The challange for the Reddit community as the site grows ever bigger, is finding them.