1) Windows.old and Win 10 UI
Deleting the "Windows.old" folder in C:\ after upgrade or repair can cause various UI elements in Win10 to stop working, eg. Start menu, Edge, and various other apps don't open.
The problem may not manifest on every occasion that Windows.old is deleted, but once it does, it is persistent and doesn't go away.
Solution: Reset / Reinstall Windows. An early sign is if Windows warns you that Windows.old is being used by smss.exe2) Applicaton Information breaks everything
Corruption, malware, or careless behavior may lead to the "Application Information" service being disabled. Once that service is disabled, it takes down with it every UI element responsible for re-enabling it - msconfig, management console, cmd, everything.
Solution: The service can be re-enabled via recovery console (there is step-by-step guide somewhere on Google, will post it later)3) Windows Firewall and Fonts
Disabling Windows Firewall breaks the Google Fonts API and the local Font management instruments - you can't install fonts due to "font file is corrupted", and websites that use Google Fonts are broken.
Solution: Enable Firewall. Check the service too.4) Animate controls and Volume Mixer
If you disable the "Animate controls and elements inside windows" option in Performance Options -> Visual Effects, the volume level indicators in Volume Mixer no longer work.
Solution: Enable "Animate controls and elements inside windows".5) Translucent Selection Rectangle and Dreamscenes
Dreamscenes may be a thing of the past, but for those who use them - "Translucent Selection Rectangle" in Performance Options -> Visual Effects may cause various issues with Dreamscenes.
Solution: If try on and off to see if it fixes whatever issues you are having with dreamscenes.6) swapfile.sys and automatic login
If you use netplwiz to set up automatic login to your password-protected account, moving swapfile.sys to another drive and/or disabling pagefile.sys may cause Windows to decline your password. Recovery console accepts your password, but the windows login screen says it's wrong, even in Safe Mode.
Solution: From the Command Prompt in Recovery Options try
wmic pagefileset create name="C:\pagefile.sys"
wmic pagefileset where name="C:\\pagefile.sys" set InitialSize=2048,MaximumSize=2048" (notice the double "\\" in the address)
If that doesn't solve it, or it solves it temporarily until reboot, then Reset / Reinstall Windows.7) Update Orchestrator service and OneDrive
OneDrive stops syncing files if the Update Orchestrator service is Disabled.
Solution: Don't disable it if you use OneDrive.8) Windows Phone GIF and equalizer
Playing a GIF animation in Internet Explorer on Windows Phone 8.1 while listening to music may alter your sound.
Solution: Restart the music player after closing the GIF.9) Battlefield 3 and 4 mouse and keyboard
BF3 and 4 may refuse to accept input.
Solution: Uninstall Riva Tuner Statistics Server.10) Disabling Windows Update service prevents installing apps from Windows Store
Solution: none that I know of.11) Disabling "Animate windows when minimizing and maximizing" causes blank / black screen on Win10 lock screen
Solution: Don't disable it. I don't want animations either, but there is currently to way around that.12) Windows Disk Management hangs / freezes when shrinking a partition
Solution: Actually, it does not. It just does not show progress in its own UI. Go to whichever drive -> Properties -> Optimize, and there, in the Windows defragmenter UI, you can track the progress of the shrinking operation in %.13) OneDrive fails to start after clearing cache and temp files
After clearing a bunch of cache and temp files, attempting to run OneDrive gave me "The application failed to start because it could not find or load the qt platform plugin windows".
Solution: Go to "C:\Users\*your_username*\AppData\Local\Microsoft\OneDrive\Update" and run OneDriveSetup.exe
After a brief reinitialization period, the app should appear in your taskbar notification area.
This post will be updated with more unsuspected dependencies as I find / remember them