To invoke Quick Look on any item in the Finder under 10.8, you no longer have to use a contextual menu, tap the Spacebar, or press the keyboard shortcut (which is Command-Y, if you're curious). If you have a Multi-Touch device like a Magic Trackpad, now you can just do a three-finger tap on the file to get your preview window.
To dismiss the Quick Look window, tap with three fingers again.
If you aren't a fan of this, you can toggle it off under System Preferences > Trackpad. It's the checkbox labeled Look up.
As you might guess from the preference name, this Quick Look functionality uses the same gesture that looking up word definitions does. So if you deselect this checkbox, you'll disable both. But why would you ever want to do that, silly reader?