what happened to smart build?

Gone, gone, gone.

Since the iPad and iPhone don't have the processing power to do SmartBuilds, that feature (like many others) was removed from Keynote 6 for OS X.

The logic is implacable.

The only stable AND useful release of Keynote is version 5 (iWork'09).

You may like to hear how Apple are planning to dilute the vitriol poured upon them by reading this:

http://support.apple.com/kb/HT6049?viewlocale=en_US&locale=en_US

Beats me why they didn't do it in the first place, but it sounds like it'll take six months to make the changes if they're to be believed...