Caveats and which upgrade path from Mountain Lion
So, I have performed my testing and taken the leap of faith albeit with a disk image to roll back to. I performed the update by way of a fresh install and data restore from Time Machine.

iMovie 10.0 is somewhat problematic. It offers the ability to read in my XAVC files but simply refuses to convert my iMovie v9 events. It'll do a handful then leave the others without mention of why. This seems a common issue. I have opted instead to use Media Converter - a free application - with a preset (Re-wrap AVCHD for Quicktime - uncompressed Audio) to enable iMovie v9 to utilise the files. I also do not like the way the new iMovie utilises an iPhoto-esque hide inside way of dealing with movies. That it doesn't convert existing events just ices the cake. My preference is for how iMovie v9, Lightroom etc work.

With regards other software I have had to switch from Zevo (no longer maintained) to Open ZFS. The latter doesn't auto-mount the drives but the switch is a blessing in disguise. It is actively maintained and utilised less CPU for the same work.
Parallels is fine as long as you've updated to the latest version. MS Office is all good. The iLife and iWork suites update but leave behind the previous versions which is a nice touch. I am yet to find any issues. I am not a major user of iCloud other than appointments/reminders which is all good.

Please feel free to tag onto this posting with notes of issues you have found. iMovie is my only item of note.