Well you can start by realizing there are people like me who treat File Explorer as a hub to all their files. Go through the databases the media players like to generate? Nope, do it through File Explorer. Calibre imports text documents and only text documents into its own file and directory structure with metadata files? Nope, I need text files next to files containing other data types There's no longer plain text files with the extension txt under "New..." but it isn't that big of an inconvenience to look up how to fix it, but imagine how much better it could be if it was trivial to add file types and templates to "New..."
The history of File Explorer windows, and hopefully sometime soon tabs, should be preserved across reboots and crashes. When there's been an abnormal shutdown, File Manager should display like Firefox, a list of all the windows and tabs before displaying anything further, in case one of the open views caused the crash or the user wants to use the situation to thin the amount of tabs they have open.
When I navigate away from a directory and then delete it and then venture back it should display in the pane files are usually displayed in something about directory not found instead of a dialog box that prevents further navigation of the history.
WinMerge 2011 allows you to start the process of comparing files and directories within File Explorer, by shell extensions on the context-sensitive menu? A good start. TortoiseGit/SVN also? Good. The processes that drive me to use both of them could seriously use a ribbon panel in File Explorer. And quit switching me to a different ribbon panel every time I enter a compressed folder. Copy and paste work perfectly fine, and I am constantly having to switch back to that panel. Clarify what is meant by folders are run in a separate process. From each other, or just the start menu, taskbar and whatever other things explorer.exe handles? It just doesn't seem like there is enough explorer.exe processes to account for all the folder views I have open.
"We will list files as you start to use them" is not a good go to event while preparing to show files. I haven't seen the message turn up for quite awhile, so it may have been fixed, but I felt like I had to point this out... again.
To parody Tim Minchin's "Not Perfect":
This is my File Explorer, and I live in it
It's where I spend the vast majority of my time
It's not perfect
When checkboxes are turned on and single click behavior is turned on and a file is clicked on in File Explorer and there is more than one file checked, nothing should happen.
This is because I keep aiming for the checkbox and somehow missing, undoing all the checkboxes I selected.
I checked two files with different extensions and clicking on one I found that the other file became unchecked and the file clicked on was passed to the program that handles it. One alternate expectation is that both files get passed to their programs. The current action removes one of the reasons the user might check more than one file.
Checkboxes should be enlarged comparable to the font sizes set at Control Panel: Display
I also find it necessary to have to use "Agent Ransack" instead of the search functionality that File Explorer provides.
Windows 98 used to allow the user to style a directory, it would be nice if that functionality was brought back. Also, floating toolbars.
There's a lot to be said about adding database-like/ish or something features to the file experience, and also make it so that people can learn about normal forms in connection with it and also not "have to" learn it.
Oh and make compression deterministic. I move a file with compression turned on from one drive to another and all of a sudden Windows reports that the file now takes up less space.