LR3-Beta : Slideshow export
In Lightroom 2 you could only export Slideshow layouts to two formats. One was JPEG, which might occasionally be handy for printing or for getting slides into ProShow. The other export possibility, to Adobe Acrobat, was a less-than-enthralling way to distribute slideshows - apart from anything else, it stripped away your carefully-chosen music.
So another welcome change in Lightroom 3 Beta is that it now lets you export to QuickTime video format which includes music.
However, this is another example of where Adobe's apparent fixation with only having two buttons results in hiding very useful features. A third button would mean users wouldn't gawp in astonishment every time one shows AutoSync or Set Camera Default, and in this case you might think Export PDF and Export Video are the only choices. But hold down your Alt/Option key and there's Export JPEG too.