As my old (6 years) MacBook was struggling to do much of anything, it was time to move to a new machine. One of the great aspects of Apple's Time Machine backup software is how easy it makes it to do this - start the new machine up, plugin the backup in, after a while your disk will appear as it was, more or less.
This page is about the more or less - the hiccups and issues encountered. To be fair to Apple, these mostly concern non-Apple software.
... naturally fails. It reports that it can't find the Google Drive folder even if you point the software at it. From previous experience, there is only one solution for this - create a new GDrive folder and let the contents be re-downloaded.
This cloud backup does something unexpected but extremely logical when transferred to a new computer - it stops backing up. The service reasons that if a new computer is attached to an old backup, the most sensible thing to do is to preserve the old backup (making it "frozen") in case of error.
This unfortunately means that backup-wise you're flapping in the breeze for a while: the new machine has to upload itself entirely, building a wholly new backup. At Backblaze's usual speeds, this could take days or weeks.
Spotlight is notorious for getting out of hand and continually re-indexing, consuming CPU. (Look for the mdsworker process as a sight of this happening.) A common solution is to exclude from Spotlight certain problematic folders that are constantly changing but contain nothing interesting, e.g. cache and temporary folders including those associated with Backblaze. However, the list of excluded folders isn't carried across in a Time Machine restore, so you have to set them again.
I chose to exclude /Library, /System/Library and ~/Library, which captures the Backblaze files. Note that to select the ~/Library folder - which is hidden normally under OSX - can be shown in the select folder dialog by pressing cmd-shift-.
Post-upgrade, Mendeley was a little fragile - crashing occasionally, repairing the database, etc. Alerted to the availability of an update, I installed the new version. Whereupon Mendeley would crash within minutes of starting. If re-opened, it would repair the database, then crash. Rinse and repair.
The solution to this ended up being the removal of Mendeley totally from the system, including any data and preference files (use Spotlight to search for them) and reinstallation. Mendeley then re-syncs your library including papers with the cloud storage, which takes forever.
Things like make, port and other unix commandline things will probably be broken by the upgrade. Time to reinstall everything.
Having installed XCode, run it and install the commandline tools (find these in Preferences > Downloads).
MacPorts is broken horribly by an upgrade. There is a migration guide offered, but this is essentially "uninstall and reinstall it". Do that.
Download the appropriate version of Onyx and use it to flush cache and temporary files.
Housekeeping programs like Hazel - which keeps track of what files are installed and created by an application - can be invaluable when deleting old and useless applications from your disk. Otherwise the cruft from long-deleted apps can hang around forever.