With the new update you will be able to post comments to shared photos. The new commenting feature is turned off by default, but is easily switched on in settings. There is a reporting feature in case you incur some sort of harassment. Comments are able to be deleted, but not edited.
There is also a new smart suggestions feature for albums. This feature allows you to match your photos with a friends photos that have been shared with you that may have occurred at the same time and place. The update is currently rolling out so be on the lookout.