There are many Google+ Photos features that are missing from Google Photos. You can remove a few features from the list: reordering photos in an album, changing the album's cover and editing the timestamp for photos.
Just click the new "edit" button when opening an album and reorder photos using drag and drop. You can also add photos from other albums.
Open a photo, click the info icon and then click the edit button next to the date and time if you want to change them.
Another recently added feature: changing the album cover. Open a photo from an album, click the 3-dot menu icon and select "use as album cover".
{ via +GooglePhotos }