Most everyone knows that you should read the NEWS file when you install a new version of Emacs. There is always a lot of new functionality and other changes that you almost certainly don’t know about. If you’re like me, though, most of it is dry and some of it you might not understand so I often don’t read it.
Fortunately, for Emacs at least, there’s a good alternative. You can read Mickey Petersen’s What’s New in Emacs 26.1. It’s really just an annotated copy of the NEWS file but it seems much easier to read and his comments provide context when it discusses unfamiliar areas.
Everyone knows about the introduction of threading by now but what other changes can you name? There’s a lot of them and you’ll probably find many of them very useful. You really should read through Mickey’s post; it’s a lot easier than trying to discover this stuff on your own.
One of the changes that is not backward compatible is apt to bite many of us. When specifying a path to a remote machine when invoking TRAMP, you now have to specify the method (/ssh:, etc.). It was optional before. Mickey gives you the workaround if you really prefer the old behavior so for that reason alone, it’s worth reading his post.