Unlike the previous built-in slideshow screensavers, Lion's slideshow screensaver is pausable by hitting the spacebar. In previous system versions, hitting any key would stop the screensaver. This is handy when people ask 'where is that,' etc.
[crarko adds: A cute little touch.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110724131342863