For me it’s the basic things that drive me crazy in Windows: the Start menu doesn’t work half of the time, and it shows web results above the program you want to run. File operations are slow and the File Explorer crashes a lot. Application windows constantly steal focus from the one I’m typing in, leading to passwords being typed into code, documents, web browsers or other unsafe places. Background indexing is constant and eats up CPU, and the file search still takes forever despite all this indexing.
These are all basic things that Microsoft has had decades to get working, and they’re all still broken. Microsoft always seem to be paying attention to anything but the quality of the user’s experience.