The things that made Git so popular are who wrote it and the fact someone built GitHub, which re-centralized things (unfortunately) to make it easier to collaborate (fortunately) via forks and pull requests. It's not like there weren't other distributed version control systems that provided much the same benefits as Git, in some cases actually doing it better for some use cases. Fossil and Mercurial had better built-in, default "developer ergonomics", for instance, and Fossil came with other benefits like an issue tracker system that gets distributed with the versioned source, an incredibly easy built-in deployment mechanism, ACID compliant version history, and so on.
In short, the GitHub pull request is the killer feature of Git relative to other DVCSes (it's certainly not the rest of GitHub, which tends toward some mediocrity of delivered value). The benefits of DVCSes themselves, absent the GitHub pull request, are of course centered around the ability to work independently and keep local versions before synchronizing with upstream.
Whatever comes along to unseat the current DVCS approach probably needs to combine integrated GitHub-like (or even better) pull request functionality directly in the developer's tooling, build on Fossil's idea of making more of the complete toolset distributed in the same way, and provide at least the independent working capabilities of current DVCSes. Add to that a new "killer feature", and see what happens.
I just don't know what that new "killer feature" would be. I kinda hope it's some kind of fully distributed routing/indexing system with BitTorrent-like shared access for cloning. While it's not distributedness per se that made Git popular, I quite like the subtly subversive character of the distributedness of DVCSes, incidentally giving rise to benefits that even centralized dev shops need so that the distributedness sneaks into everything. Empowering a greater diversity of innovation seems, to me, like a clear win.
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