I keep a checklist of user interface requirements in my head while I work. But if I had to write them all down, it would look a lot like this excellent article, Visual design rules you can safely follow every time.
Interface design becomes far more complex once you’re working on the specific features of the system. But using the above resource, there can be a solid foundation to build off of, and make developer (and therefore, user) experiences easier down the line.
I’m especially a fan of the Dark Mode guidelines. In the early days there was a lot of bad Dark Mode design out there. This feature depends on the system being well-designed: if the layout is organized and clear, with proper importance hierarchy, the easier it is to go Dark Mode. Just don’t screw it all up by making the background lighter than the foreground.