Reddit started out with a nerdy community that bootstrapped excellent content. They added features slowly, focusing on quality and the moderation system. The idea of sub-Reddits allowed groups to form, each with its own “rules” to ensure the site remained useful to everyone as it grew.

I think it had to do with curation. With SU, it was totally random, but with reddit, you got a list of stuff other people liked.

And in fact, the SU functionality is part of reddit:

http://reddit.com/random

Dopamine manipulation via comment karma.