I read through a pretty comprehensive OS X Photos article by Rene Richie at iMore: http://www.imore.com/photos-os-x-yosemite-explained
It goes into as much detail as is available right now regarding Photos in Yosemite. The section that most concerned me was:
In terms of organization, Apple has shown that the same, automatically generated Years, Collections, and Moments views that currently exist in Photos for iOS will be implemented in Photos for Mac, as will Albums. How existing iPhoto Events get mapped, be it to Moments, to Albums, or to something else, remains to be seen.
So it's just not known at all what's happening to well-organized iPhoto libraries once the transition happens. It's a little maddening that we're not only left wondering what the new solution will be, but also for such a length of time! Why on earth is the roll out of the new Photos system happening piecemeal over iOS 8 (Sept 2014), then Yosemite ("Late 2014"), and THEN OS X Photos ("Early 2015")? Why on earth won't Apple explain how the transition is going to happen? If I need to re-map my Events into Albums in order to preserve them, I'd sure like to know sooner than later.
I'm most concerned about what iCloud Photo Library is going to mean to users like ourselves, with very large iPhoto libraries. Will iCloud Photo Library be the ONLY way to sync photos to iOS in the future? Will iTunes syncing vanish? And if so, does that mean if I've got a multi-GB photo library on my Mac, I'll suddenly have to PAY for extra iCloud space in order to sync the whole thing to an iOS device? That's absurd if true. But Apple has been so slow to release any meaningful info about the future of photos on the platform that I'm just left to stress and speculate.