UPDATE: Dupe detection now works for me. I've uploaded a number of images twice and it did not accept the second upload.
I am essentially cleaning my Google+ Photos library manually of around 50,000 pictures. Taking the (200) Takeout archives, going through them manually to de-dupe and remove JPGs if there's a RAW version, too, etc.
-- Initial Thoughts --
I'll miss the comments on photos. I'll also miss the editing tools that came with Google+. Worse, since there's no such thing as a public albums page anymore, I'll have a harder time browsing others' (friends') albums.
* You can't add location information to images. Picasa web albums, which still is there, can do it and it'll reflect into Google Photos, but that's an extra roundtrip.
* Google Photos's new "Assistant" has a fun time taking everyt RAW image I've uploaded and turning it into a motion video with the associated JPG in which barrel distortion is corrected.
* No face detection for me. None. Zip. Zilch. Also no way to tag new faces on the web.
* If I create a sharing link for one image it'll link to an almost blank page with a thumbnail of my image. For any single image share it'd be much better to link to the image directly.
* Collections have an upper limit of 2000 images. I took that many of my kids in their first week.
* The editor has seriously lost its luster going from Google+ Photos to Google Photos. No more layered editing, no more manual corrections. That's also sad and mad.
* Google Photos -> Google Drive is a one way sync. There's no way to move a Google Drive saved photo into the Google Photos folder and have it shown in Google Photos. That's highly inconsistent.
* Same for deleting. Deleting a photo in Google Photos will delete it from the Google Photos folder in Drive, not the other way 'round, though.
* Uploading a 13MP photo with "Originals" setting will count against your quota. Uploading it with "High Quality" will store a compressed image and not count against it.Uploading any photo, even if it's under 16MP with "High Quality" setting will compress it.
* Same as with location, all metadata needs to be edited in Picasa Web, can't be edited in Google Photos.
* No "cast" option in the new Google Photos app for Android.
* Any action will result in a scrollback. Example: scroll down to April 2014, open a picture... have to scroll back down to April 2014.
* No more drag selection. In Google+ one could rectangle select with the mouse, this is gone.
* Edited images are saved as new images (defined by the edit date). Among other things, this means that we have no indication that there are edited versions of images unless we remember editing them --
*Unable to share gifs from Google Photos (the app) to G+ and have seen others report the same --
[Will expand as it happens]