The reason why I think it's iOS-ification rather than an OSX-ification is because "Photos" seems less like a standalone app and more like a necessary utility to manage content created by the Apple ecosystem. iPhoto never really felt like that.

iPhoto was part of the iLife suite that were standalone apps to manage the different multimedia demands of your life, mostly from non-Apple digital cameras. It was optimized around editing and organizing your photos. "Photos" on iOS was a barebones app required by a device where camera functionality is built in, intended primarily to manage photos created by the iOS device. Over time both of the apps have borrowed features from each other and grown together until it made sense to converge into one app.