View -> Preview Pane.
Adds a pane in Windows Explorer that shows a preview of the file. I believe it was on by default back in the XP days, but since then it's just defaulted off.
EDIT: Huh. I knew it was there, but I never turned it on. It...actually looks pretty awesome. You can read an entire PDF in the preview pane.
Apart from that, Windows Explorer is definitely above the Finder that has been stalling for years now...
FWIW the Windows Explorer preview is much improved compared to what it used to be - it's not a little box, it's the entire right side of the Windows Explorer window. There's ample space to actually read a PDF, you have pagination controls, you can read spreadsheets, etc. I wasn't sure it even still existed until this thread, so thanks for that. :)