One thing I found right away is the Yosemite upgrade killed my installed Postgres. I think Apple's upgrade process did a search for certain postgres folders (for temporary files, no data files are deleted) and deleted them, probably assuming they were for their Postgres and that they would be reconstructed later in the upgrade process. I added some repair instructions on my Postgres page.
Well, I was forced to upgrade to Yosemite this week. I held out on Mt Lion as long as I could. Even shortly before the release of El Capitan there were signs of the impending demise of Mt Lion at work: certificate problems on some sites, installing Mt Lion from scratch (couldn't) to try to fix a slow Mac, one critical package we use "deprecated" Mt Lion support in their current version.
I suspect Apple will stop providing security updates for Mt Lion, we'll see in their next update. I haven't decided yet if I will support Mt Lion if Apple does not continue security updates, it's a nice solid system but the certificate issue makes it impractical for general use.
I was going to start looking at GDAL 2.0 this weekend, but I need to check that my development environment is working properly first. Shouldn't be too hard.
Almost all of the frameworks have been updated.
Many python modules have been updated:
NOTE: Numpy and Scipy are now 64bit-only. Do not update these if you use them in GRASS, because GRASS currently runs python in 32bit mode.
After some distractions and chasing a phantom problem, Postgres 9.4.4 is ready. PostGIS and pgRouting extensions have been rebuilt for the new Postgres. Make sure to read the upgrade instructions.