No idea what the nitra application is, but all releasenotes can be found here
This is a rather broad question, as also mentioned by others already.
What are the main topics you're evaluating in your migration decision?
Mendix 6 has many new features and improvements like fixes, security patches and support for recent databases & Java stack.
* Responsive UI & Offline mobile [https://www.mendix.com/application-platform-as-a-service/responsive-ui-framework/ & https://ux.mendix.com]
* Rest Consume (Mx 6.6.0+), Publish OData [https://www.mendix.com/application-platform-as-a-service/open-platform/]
* CloudFoundry support [https://www.mendix.com/cloud-foundry/]