One of the big ticket items when it comes to SharePoint 2013 is the new Design Manager which you’ll find under Site Actions. From here you can upload design files (AKA: Your own custom .HTML) straight out of Dreamweaver. In fact you can connect and edit your files in Dreamweaver.
SharePoint will make the connection between your file(s) and the master page, and then it’s editing as normal for the most part.
The other major addition to the customization space is “Channels” which allow you to specify different master pages for different devices. Same concept as media queries. Maybe not the optimal solution in 2-3 years when folks are well underway with responsive design but it is a step in the right direction.
Another new item is the “Snippets” which are pieces of code to insert things like the top navigation bar, search menu, vertical navigation bar.
Rather than regurgitating what someone else has already graciously written, I will simply point out a post that I found to be a great introduction to the Design Manager and how you can leverage it to create custom SharePoint Master Pages in SharePoint 2013.
Another look at some of the Design Manager Features: