Full-screen Quick Look!
The shortcut we all use for Quick Look is Space Bar, but it has another shortcut that I didn't even know about. It is "Command - Y". The shortcut can be useful if you want to go directly to full-screen Quick Look. You just have to add "Option" so that it is "Option - Command - Y" and that new shortcut will go directly to full screen. If you select multiple photos, it will also launch a slideshow.