Changesets, the op log, first class conflicts, no staging area.
JJ might produce commits that can be stored in Git, but the affordances are different. If Git wants to adopt them, it becomes no longer Git.
On the other hand, I'm happily using JJ while everyone I collaborate with is using Git. JJ doesn't need to "win" to be useful, it just needs to be useful enough that the people who maintain it continue to maintain it.
JJ might produce commits that can be stored in Git, but the affordances are different. If Git wants to adopt them, it becomes no longer Git.
On the other hand, I'm happily using JJ while everyone I collaborate with is using Git. JJ doesn't need to "win" to be useful, it just needs to be useful enough that the people who maintain it continue to maintain it.