Our site has lots of fine-grained permissions. We're currently running 2010, but are in the process of looking at moving the DB's to 2013. I've just figured out how the "Shared With" button replaces Item Permissions. for 99% of what we do... the old behavior is what we need. Is there any way to force the Shared With button to pop open the Advanced view?
Selecting the Item > Share > Advanced is how permissions at the item level are managed in SharePoint 2013.
To review the unique permissions from the exceptions dialog box that can be found at the Site or App/list/library level permissions page. From here you can see the items that are unique and directly manage them if need be. Note this part of the permissions yellow information ribbon only will appear if this is true.