The one difference between the apps that changes is where iPhoto automatically created Events for you (and sometimes you needed to tweak them to get pictures where you wanted), in Photos.app you need to create Albums on your own after importing pictures (or taking a batch of pictures, etc.)
But honestly, if you were the type that micromanaged your events and kept them "clean and organized" in iPhoto, that won't change in Photos, you'll just do so using Albums.
This is the nub of the problem for me.
'Collections' > 'Moments' is arbitrary and can not be edited.
I wouldn't describe how I used 'Events' to manage photographs as 'micromanaging' though.
For example, if I went on holiday, I'd group all the photos (including iPhone pics) into an Event for that holiday.
Okay, I could do that with an album, but there is no way of telling whether I've got all photographs related to that holiday in the album - under the old system, if I was going through an adjacent Event and found a photograph that should be in the holiday Event, it would stick out. Now there's no way of telling, because Albums are not the primary way of sorting.
'Moments' doesn't do the same thing, particularly if you're using anything other than a GPS-enabled camera like an iPhone.
To bring a useful functionality, photographs would have to be able to be moved between 'Moments' and the user have the ability to rename 'Moments' - currently I've got some that are named after the nearest road, which is just daft.
To make 'Collections' useful, being able to rename them would be a start, but to make them useful it would be handy to be able to select which 'Moments' are allocated to which 'Collections'.
'Albums' simply don't constitute a useful replacement - they're handy, for example, for putting the best selection of photographs from a holiday but they're not a primary means of organising a big photo library.
Basically, the new Photos app is designed for a user who has little interest in their photography collection other than it displaying the photos they take with their iPhone.