- If you are a homebrew user, you might want to backup your /usr/local directory and then remove or rename it before running the upgrade. This is, reportedly, why my upgrade was stuck on "about a minute remaining" for almost 20 minutes; basically Yosemite will go through that dir and "fix" a lot of your hacks.
- For the same reason, backup your PostgreSQL data. If you don't, you'll have to fix it as reported on this StackOverflow question, i.e. recreating some dirs under (guess what) /usr/local.
- iStatMenus 3.27 was branded "incompatible" and disabled at first boot. After manually launching it, it had to "upgrade some components", but it seems to have been working fine since then. All its configuration was lost though. You might have a better experience with (paid upgrade) iStat Menus 5.
- IntelliJ PyCharm 3.4 seems to crash on startup. Apparently this is a known problem for all JetBrains products at this time (as commenter Bardia confirms below), due to them using Java 1.6 which is not officially supported on Yosemite. To fix it, I opened /Applications/PyCharm.app/Contents/Info.plist and edited the JVMVersion key from "1.6*" (i.e. "any 1.6 version") to "1.6+" ("any version greater than 1.6"). This found the 1.7 Oracle JVM I had previously installed, and launched successfully. However, it looks like the standard interface fonts now look bad (but not the custom ones used in actual editor tabs, somehow). I'll probably have to read through the relevant thread in the IntelliJ forums at some point.
- my not-regularly-updated homebrew was broken because it expected Ruby 1.8 to be available, whereas Yosemite ships only 2.0. To fix it, I successfully followed the instructions on this comment, just running the listed commands with sudo (because my homebrew is oldish and sudo-ed and I can't be arsed to reinstall and I'm a bad boy like that).
- The upgrade freed about 10GB of space, which is nice but makes me slightly nervous about what was removed.
- "Dark mode" is gorgeous, but the the new system fonts and overall "flatness" just make me sick. Apple Mail now looks sad. Guess I'll have to get used to it for now.
- Animations feel a bit glitchier than before, on my beefy 2012 MBPr (2.6 i7 with 16GB RAM). It took about 5 minutes before I got the first system crash (pointer still moving but desktop completely unresponsive). Apple must be working hard behind the scenes to sell me a new laptop. EDIT: I've found the way to get pre-upgrade performance -- System Preferences -> Accessibility -> Display -> Reduce Transparency should be ticked. It makes a HUGE difference.
- The upgrade didn't seem to make much of a difference with my HDMI problems (I have a new external monitor that can do 3840x2160, but it stopped working via HDMI earlier today, well before the upgrade. Hopefully it's a cable issue.)
I will likely update this post in the following days, as I experience more problems.