Last year at WWDC, Steve Jobs noted that there were "many" features they weren't showing us yet in Leopard. This year, we should (I think) see those features, as Apple will be releasing a "feature complete" beta at next week's WWDC. On Monday, Steve will take the stage to (presumably) discuss Leopard and other things of note. Anything mentioned during the talk is public knowledge (unlike the rest of WWDC, which is under non-disclosure), so I thought it might be fun to speculate for a few days on what you think the single most interesting "hidden feature" might turn out to be...
I'll be interesting to see if anything on this list (beyond zfs, which seems to be a known fact now) is actually in the Leopard beta.
-rob.

