The first was when posts started showing thumbnails. This made image content much more viable. Unfortunately, while there's plenty of awesome image content, it's also one of the lowest-effort routes for "shitposts" (the definition of which is left to the reader).
The second was the growth of subreddits. This allowed the community to split and silo itself. As a clichéd example, you can pick your left-wing politics subreddit or your right-wing politics subreddit, and you never have to mix with the other, bad one!
And the third was the death of Digg, which sent a huge influx of people Reddit's way. That one's got a lot of folk mythology behind it and little hard data, so I'll say no more.
