I suspect one of the network effects is simply that people have Reddit accounts and know how the site works; it's not yet another site to sign up for, for a small-ish community of people interested in a thing. I think that's the sole reason, for instance, /r/rust works as well as it does; philosophically, the community doesn't align well with Reddit as a whole. I suspect that it's not users of any particular other subreddit (not even /r/programming) who are there, but people already using Reddit for many many other things. It's become one of the bigger discussion forums for the language, especially since the closure of the rust-dev mailing list.
If you can build a decentralized Reddit-like system with no per-community account system and also with no spam problem, you may stand a decent chance at replacing it.