One of the big limitations of Field Security in previous versions of Dynamics CRM was that it was only available on custom fields and not on out of the box fields. In Dynamics CRM 2015 however Field Security can now be enabled on most out of the box fields, with only certain system fields such as the primary key/primary field or status/status reason not having Field Security as an option.
Another limitation of Field Security that has been removed is on types of fields Field Security can be used on. In the past Field Security could not be enabled on field types such as email fields or address fields, now for a custom field Field Security can be enabled for all field types, including new types such as Rollup or Calculated fields.
Lastly to help in administering which fields are enabled for Field Security, in the fields customisations area there is a new Field Security column added to the view.