The space bar is the key you need to remember if you are using Quick Look. When you are browsing files in the Finder, hitting the space bar will open the file in Quick Look. Since Quick Look really is quick, this is a great way to scan files if you are looking for something in particular. Once Quick Look is opened from the Finder, hitting the space bar again will close it and put you back in the Finder.

If you are looking at a multi-page file in Quick Look and you are go into full-screen mode, you can use the space bar to page down in the document. Holding down the Shift key while hitting the space bar will page the document up.