Being able to select any file in any window (explorer but especially in file upload dialogs) and by pressing spacebar see a quick preview of the file.

There's several tools already that copy this popular MacOS feature but they never fully integrate with the OS well. They usually only work in explorer.exe but this feature is most useful if you want to upload a certain file to a website and you're trying to see a slightly larger preview to make sure you picked the correct version of a document or logo.

Check out #99 which had some great comments too.