As far as specifics, one of the first tasks I wanted to perform was to survey some of the NoSQL databases with some test cases that I would build in Python.
Mongo had a good 3.x driver and I worked with it. Great... but then these drivers all put me into a brick wall trying to use 3.x: couchdb, pycassa, happybase, and cql. There were numerous others that I tried along the way, but I don't have notes on all the failures.
Now, rather than trying to be 3.x pure, I don't even bother checking. I'm just using 2.7 for this project.