Well the short answer is there is no new functionality added to InfoPath 2013 from InfoPath 2010.
Here is a link with more info:
However for writing form code - Now it uses full Visual Studio 2012 instead of VSTO which is good change.
Reason: The VSTO with InfoPath 2010 was limited to using .Net 2.0 framework
whereas the InfoPath 2013 creates the Form code in .Net 4.0
Hence that limitation of API while writing InfoPath code is overcome now.