Since they aren't sharing any of these details, we have no choice but to assume there are none. In fact, their exact reason for not using Git was "it also provokes strong antipathies". That doesn't even _imply_ technicality on any level.
Either way, I couldn't care less personally. The only people they're hurting are themselves, as a gateway can be created between Git and Mercurial. All the projects I work on right now use SVN, and I just use git-svn for everything.
The Python team purporting to do DVCS research and then saying their decision was made on a gut feeling seems disingenuous at best.
Sure, I'm biased because I work for GitHub, but this decision would have felt like much less of a slap in the face had they just said: "We're switching to Mercurial because we like it and it's written in Python, case closed."
Python has a history of fussy programmers. Remember the web framework wars of a few years ago? Everyone complained that there were too many choices, picked favorites to champion and nitpicked at the alternatives (which were not actually bad), and nobody could sleep at night because there were alternatives and nobody knew which one was right. Then Guido announced that Django was right, and the lost souls breathed a sigh of relief and did the Django tutorial, and the folks who were already using Turbogears and Pylons productively continued to do so. Django isn't necessarily the best Python web framework for everyone, but it's rarely a bad choice.
That's pretty much what happened this year with DVCSes. The Biopython project was stalled on a CVS-to-SVN transition for over a year, for example, until some grenades were thrown and tears were shed and finally Git was picked because a few people liked it, nobody really had a problem with it and all the other options were triggering nasty blog posts. Case closed. Sometimes we just need a quick slap to shut us up.
And I can understand how you would wish they used git, but if you're taking this as a "slap in the face" at all, I think you're reading it wrong. They did some research on various similar tools and picked one.
My problem is not with their decision, but with how they made it. They are taking the "cathedral" approach, rather than the "bazaar" one.
Also known as "weasel words": http://en.wikipedia.org/wiki/Weasel_word
This seems illogical at best. They chose not to use Git because there are a lot of people that dislike it? This decision should have been based on technical details, not on feelings. Anything else is doing a disservice to the developers and to the community. How many other VCS's were passed up because they had a lot of haters?