Perhaps the most important point is the continued availability of security updates. Eventually MS will stop supporting Windows 7.
The Windows 10 UI is a bit different from Windows 7, although I think it's much better than Windows 8--far less "tablet oriented". The upgrade was fairly painless. Everything worked, except for the driver for a Focusrite Scarlett 2i4 audio interface, and in that case Focusrite came out with a (initially) beta driver and then a release driver.
Everything, Adobe CS6 and (at that time) LR 5 worked fine. Additionally NIK, On1, and DxO worked. SilverFast 8 ran without issue. My Epson 3880 printer driver worked fine, too. Even my very old graphics tablet, an off-brand and not a Wacom, installed without issue.
You asked specifically about photo editing, but SoundForge and a raft of audio software (Studio One v3, Native Instrument's Kontakt, IK Multimedia's Miroslav) worked just fine. In fact, all the software I had installed earlier just continued to work.
About the only change I ran into was the default browser is Microsoft Edge, and it does not support Java applets. However, IE is still installed, and it does continue to support applets. Of course Chrome quit supporting that API some time ago, so I wasn't surprised.
It doesn't see to run any better, but it also doesn't run any worse. At least the dreaded "blue screen of death" -- see Focusrite driver above -- is prettier.
Tom Frerichs