- 32-bit apps will no longer be supported at all (
currently that still includes Acquia DevDesktop – for running Drupal development sites)
- the default shell is going to change from Bash to Zsh
- scripting languages won’t be installed by default (Apple gave Python, Ruby and Perl as examples in a support document, but didn’t specifically mention PHP). It’ll mean Homebrew will need to be installed differently. Presumably it won’t be an issue if you’re upgrading a previous macOS version, but, also presumably, it will be if you’re using a new computer, even with Migration Assistant.
- iTunes has been removed and the Apple Music replacement apparently doesn’t support column view of your music library.
- John Gruber writes that Catalina is a pain to use due to (a) bugs but also (b) excessive permission alert dialogs.
(As usual, my own policy is not to upgrade to a new macOS release until 9-12 months after it’s come out, to allow everything to settle. I write this – post updated Feb 2020 – on a machine that’s still running High Sierra, which should still get support until late 2020 – although again, Apple have never announced support lifecycle dates.)