I'd be willing to bet the sub-reddit idea was heavily emphasized since that's arguably the only novel idea that reddit had, but it's a good one that allows it to not turn to shit and stave off eternal september for as long as possible.
Maybe I'm super naive, but that really seemed like something that was added when people wanted to filter posts, and then later these filters got styling and a community sense.
Maybe it was the plan all along and they just gradually implemented it in a way that felt organic and user driven.
But I agree with you, the subreddits are brilliant at what they do. The concept of merging all your favorite BB into a single stream is fantastic and pulling it off is even more impressive.
But early reddit did not look to me like anything that was aiming to be more than a digg clone that used basic text layout to scare the n00bs away and then it just gradually evolved into this crazy huge community.
Cannot find a source at the moment (I think it was a podcast), but I recall that Joel Spolsky was influential in getting them to adopt the idea of subreddits.
