Having gone through this a few times now, I have a few additional insights.
If your machine is a tablet or hybrid drive it will have specific touch configuration which will be very difficult to recover in the event of a bad migration.
In Windows 8/8.1
create a USB recovery drive. This will give you a drive which you can use to completely recover your machine from a borked install.
Other things. I fall into the situation where I have keys which work in Windows7/8/8.1 but do not work in 10. For me I have to do any updates in the original OS. For instance with Mrs S hybrid tablet, which came with W8.1, I used the "Add additional features" in the settings to use my Pro key to upgrade her machine before upgrading to W10.
This allowed me to get full W10 pro on the machine which allows me to do things like remote controlling it from my desktop machine with a big screen.
If you have anything odd, or weird, or wonderful, in your config, then get it out before you upgrade. Personally I run two graphics cards because I haven't been willing to buy an expensive enough one to run as many monitors as I really need. This is causing me quite a bit of pain with having to switch off advanced features in IE (doesn't display the screen), Firefox (Crashes continuously and had to start in recovery mode) and even then stuff like the Ryanair site won't load in IE.
This is not so relevant for upgrades but more for reinstalls or new W10 machines.
For those of us who may use Multiple mail accounts for different things, for god's sake DO NOT let Microsoft bully you into using that account to log into your machine.
During setup it asks you to log in with an existing mail address. There appears to be no option to do anything else, but there is an option to create a new one. When you select to create a new one,
In the new screen you see the options to create a new account, but right down the bottom in small text is the option to skip this. Windows then allows you to log on with a local account.
Why not do what MS wants? Because it will default every single mail and messaging service to that account instead of asking you which one you want to use and, in some cases, it's virtually impossible to stop it doing so.
MS office365 and W8/10 and OneDrive.
I have this issue that I can't add my personal OneDrive to the OneDrive app when I install Office365. I can use the Office365 one but I can't add the personal one. If I don't have Office365 installed but a normal vanilla Office 2013 installed, I can add OneDrive and then add the business account to it.
This is a minor irritant and I guess most people will never see it. But it's something I'm seeing.
I'll add more as I run over them.