If I recall correctly on the day (weekend?) that Linus tried monotone, the then-current release of monotone had some diagnostic/debugging/profiling code compiled in which caused it to have superlinear runtime for some computation or other. Correct me if I'm wrong, Graydon, as I think what I'm recalling is from something you wrote shortly thereafter.
It's one of those "for want of a nail the horseshoe was lost" kinds of moments in history -- if monotone had been fast enough for Linus to use at that time then presumably he never would have invented git.
And while *most* of the good stuff that the world has learned from git is stuff that git learned from monotone, I do feel a bit of relief that we have git's current branch naming scheme. Git's approach is basically to not try to solve it, and make it Someone Else's Problem. That sucks, it leads to ad-hoc reliance on DNS/PKI, and it probably contributes to centralization e.g. github, but at least there is an obvious spot where something better could be plugged in to replace it. If we had monotone's deeper integration into DNS/PKI (http://www.monotone.ca/docs/Branches.html), it might be harder for people to understand what the problem is and how to change it.