Photos App / No Books or Slideshows
9 to 5 Mac reports iOS 8 Photos doesn't support books or slideshows. I've added this item to my Aperture v. Lightroom v. Photos comparison chart. I've not used the slideshow capabilities of Aperture, though did occasionally create books (even if only for PDFs).
This isn't a deal breaker for me personally for Photos, although it's a significant drop in the feature set. And one I don't understand – I'd think the masses, Apple's obvious target with Photos, use these features in iPhoto a lot.