These things cost me some time or annoyed me, and I just thought I'd document them in case it helps someone else, or in case someone else knows a cure for some of them.
✓The MediaCreationTool for creating the installation DVDs contains an initial screen with 3 drop-down selections. However, the user is highly expected to select these from top to bottom, because higher choices cause lower choices to be altered from the user choice back to some default. This is clearly a violation of the idea that choices presented simultaneously can be selected in any order. If there is a required order, that should be enforced in software, rather than by changing previously made user choices. I wasted bandwidth and a DVD by downloading the wrong version of Windows the first time, because I selected the bitness while trying to figure out the difference between Pro and Pro N.
Delivery Optimization for Windows Updates sounds like a great new feature! With 4 Windows PCs on-site, downloading all of the over the broadband connection seems wasteful, so having them borrow Updates from each other sounds efficient and good. However, there seems to be no control of the caching... "for a few days"... some of the machines here don't get turned on very regularly, so might miss such caching. It would be better if one machine could be designated as the Update master for the site, and be told about the other machines by name and number and type of Windows (Pro, Home, bitness), and obtain and keep the relevant updates until those machines acquire them.
This leads to the next limitation, mentioned in the Delivery Optimization FAQ, regarding metered connections. The answer in that FAQ implies that only WiFi connections can be treated as metered. While it is true that WiFi is the type of connection most likely to be metered (via a cellular hotspot), it seems that USB WiFi dongles are counted as Ethernet (at least on one machine I have that uses one), and there do exist WiFi routers that use WiFi as their Internet source, but have Ethernet ports for connecting local devices (I have one of those I use when traveling in the RV with multiple devices). The automatic detection of WiFi is unfixably flawed to the variety of devices that can convert (metered) WiFi into other connections, and the restriction of metered connections to WiFi connections is inappropriate. There are metered Ethernet connections as well.
VNC mouse actions seem not to work in certain control panel dialogs.
Win10 upgrade turns on Avast! web scanning of HTTPS connections via MITM proxy. Firefox detects, prevents this from working, but gives a stupid message, so you don't know why the problem is really occurring. https://support.mozilla.org/bs/questions/1075167 To be fair, the root cause of this problem is possibly Avast!, rather than Microsoft, and the diagnostics from Firefox could certainly be improved.
If you have Classic Shell installed on Win7 (or probably Win8.1) and you plan to upgrade to Win10, uninstall Classic Shell first. It doesn't survive the migration, and apparently its tweaks to Windows causes problems for the Start Menu of Win10. Cure can be to install and then uninstalled a new version of Classic Shell on Win10. If you really want to use Classic Shell on Win10, you can then reinstall it again, after the upgrade, or after the cure. But consider using SyMenu instead, if you mostly want Start Menu functionality. While this could be considered a Classic Shell problem, enough people were using Classic Shell and other similar programs to work around previous stupidities in previous versions of Windows, that appropriate handling for them (at minimum, uninstalling them properly) should have been included in the Windows 10 upgrade process.
Win10 has smaller borders, making it harder to find the spot to resize windows. NiftyWindows is a pretty nice solution, but it seemed to have many features I'd never use, so I made a modified version with just the RIGHT_BUTTON+DRAG features retained.
Win10 has made it extremely difficult to distinguish which window has the keyboard focus, also known as the "activated window", or "top of the Z-order". If you turn on an accent color, that is used as the 1px border around the activated window, which otherwise seems to be transparent. Also, if the title bar text is showing, it dims from black to grey, and the minimize, maximize, and close buttons do likewise. Both of these behaviors are subtle enough to overlook. These instructions claim a way to almost make things good again.
Win10 seems to have screen drawing issues, leaving garbage behind when changing settings in the settings screen, and occasionally window borders as well (reverse Ls). The latter does not vanish on refresh. They will vanish in Task View, but come back on exit of Task View. And sometimes things flash back and forth from before to after, pieces of things will jitter or flash on and off on the screen. Very distracting. I wondered if it is a heating problem, but it even happens with my big fan running.But maybe less frequently when cool.
Windows 10 changed something that causes Evernote to display ✓ characters as the "can't find that character in any font" box, whereas before the upgrade, it displayed it fine. Interestingly, in the "snippet" view, it can still display the ✓ character itself. ✓ is U+2713. So perhaps there is an existing Evernote bug that Windows 10 is exacerbating here.
Windows has a dialog (Control Panel / Sound / Sounds tab) where many different sounds played by the system when various events happen can be configured. Once a particular sound is selected, the Browse button allows you to choose a file from anywhere on your computer. However, even though the Test button will play that sound once selected, the actual Windows event will not, unless two conditions hold: the file is located in the %windir%\Media folder, and it is a stereo .wav file (mono doesn't work; not sure of all the specific media characteristics required).