What has changed significantly
New commenting model
Comments now work more like comments in other Office applications. There’s a commenting panel, and comments can be threaded. For details, see Review and add comments to Visio diagrams.
Existing comments in a document are upgraded when you save a diagram in the VSDX file format.
The Database Model template and UML (Unified Modeling Language) template have been reworked
Visio templates for Database Model diagrams and UML diagrams have been remade with new shapes. The new templates are more flexible, but offer no reverse or forward engineering or other solution support.
You can still view and print UML diagrams created in previous versions of Visio, but editing is limited and changes to the diagram might prevent the file from being editable in earlier versions of Visio.
What has been removed
Pages can no longer be rotated. Rotated diagrams from previous versions of Visio will have the page rotation removed when the document is opened. Saving the diagram removes the rotation from the original file.
Save as Visio 2003 XML formats
You can’t save in the Visio 2003 XML formats, but you can still open Visio 2003 XML files.
Save as Visio 2002
You can’t save in the Visio 2002 format, but you can still open Visio 2002 files.
Viewing a SharePoint task or issue list as a Visio PivotDiagram
You can’t create a PivotDiagram from a SharePoint task or issue list, but Visio will still synchronize data from SharePoint 2010 task lists and issues tracking lists.