That said, building a version control system seems like a problem similar to a social network: the network affect causes an enormous amount of friction. i.e. most people don't want to use it until other people are using it. A vicious cycle.
The fact that it's compatible with git as a backend is really great though, and is probably the key to driving adoption. However I have several immediate thoughts that come to mind that cause me hesitancy. Answers in the README.md would be super helpful for marketing to me (and deep apology if it's there and I missed it):
1. How does it look to others working on the repo using just git? For example, "when the working copy is automatically committed," where does it go? is there a remote branch that someone could see if they `git fetch` while I'm working? I often put all sorts of things in the code that I don't want to have committed, ranging from harmless (a bunch of personal comments or `IO.puts()`), to very important (API keys and such that I'm testing with. I always move them to env vars before committing but for first-pass testing to prove the concept I "hardcode" them at first).[1]
2. Similar to "how does it look to others," what sort of burden does "all operations you perform in the repo are recorded, along with a snapshot of the repo state after the operation" put on the git backend? If I'm hacking on an ffmpeg script and I (temporarily) copy a 4GB mp4 file into the working directory so I can easily run `./mycode video.mp4`, does that whole thing get committed, potentially every time it changes? That could easily turn into a 40GB set of changes.[1]
3. Do you have to use the jj backend to get all the feature? For example, with a git backend could you get the two things mentioned above and also "Conflicts can be recorded in commits" as well?
A quick section in the README.md about which features require the jj backend instead of git would be super helpful, and if written well could answer all of the questions above.
To sum up my comment in a TL;DR: To really sell me on a project like this it has to work with git, and being able to quickly tell which features I can use with git, would make this substantially more interesting to me.
[1]: Related: https://xkcd.com/1172/