Windows 7 Preview Pane
First off, I didn't even realize that Vista had a Preview Pane feature, but today I had the need to quickly glance through several Word documents and was trying to find a way to quickly preview the contents. I noticed an icon within Explorer in the upper right that looked like a preview button and wha-la! I haven't tried for other file types yet but for Word documents the experience is fantastic.
Gizmodo has additional info here: http://gizmodo.com/5140434/win-7-tip-windows-explorer-preview-pane-works-much-better-than-vistas