This document describes things to keep in mind when moving from v5 to v6. It does not list enhancements nor does it talk about compatibility concerns introduced by earlier versions (for this, see their respective compatibility documents).
Version 6 offers a better config language and some other improvements. As the config system has many ties into the rsyslog engine AND all plugins, the changes are somewhat intrusive. Note, however, that core processing has not been changed much in v6 and will not. So once the configuration is loaded, the stability of v6 is quite comparable to v5.
Traditionally, this property did not only return the textual form of the pri ("local0.err"), but also appended the numerical value to it ("local0.err<133>"). This sounds odd and was left unnoticed for some years. In October 2011, this odd behaviour was brought up on the rsyslog mailing list by Gregory K. Ruiz-Ade. Code review showed that the behaviour was intentional, but no trace of what the intention was when it was introduced could be found. The documentation was also unclear, it said no numerical value was present, but the samples had it. We agreed that the additional numerical value is of disadvantage. We also guessed that this property is very rarely being used, otherwise the problem should have been raised much earlier. However, we didn't want to change behaviour in older builds. So v6 was set to clean up the situation. In v6, text-pri will always return the textual part only ("local0.err") and the numerical value will not be contained any longer inside the string. If you actually need that value, it can fairly easily be added via the template system.
If you have used this property previously and relied on the numerical part, you need to update your rsyslog configuration files.
The plugin interface has considerably been changed to support the new config language. All plugins need to be upgraded. This usually does not require much coding. However, if the new config language shall be supported, more changes must be made to plugin code. All project-supported plugins have been upgraded, so this compatibility issue is only of interest for you if you have custom plugins or use some user-contributed plugins from the rsyslog project that are not maintained by the project itself (omoracle is an example). Please expect some further plugin instablity during the initial v6 releases.