I just came across a piece on icons next to menu items. One thing it doesn't address is the added clutter, and the alignment issues between a piece of text and its icon (and menu items with an icon and those without them). I'm not sure why they aren't used very much on MacOS, although support's been there since at least System 6. Safari has an icon next to its bookmark items, and the dock displays little window icons next to the entry for a window in the dock menu, and there's files' icons in the dock menu for a folder (like in ye olde days in the Apple menu). If you insist, I'll also count menu icons, but there aren't many more. Windows is different here. Lots of icons next to menu items, and you also see that on ported apps occasionally. I wonder whether that's just historical, pragmatical (i.e. having to design an icon for every menu item is too expensive, and reuse isn't desirable here) or whether there's actual usability research behind that.