Well, there is a bit of documentation regarding out testing here: http://http://docs.jboss.org/ironjacamar/developerguide/en/html/testing.html - which of course could be expanded. Especially the "split" of test cases described in 5.1.x is important to keep in mind.
As to the most important parts - coverage of the deployment metadata is important from an end-user point of view, so we are sure that it works, and that we can provide examples for the users on how to configure their resource adapter / deployment.
From a container point of view - the most important part would be the connection manager (and friends) - especially the local / XA transaction based one.
I agree that there may be certain classes / methods that don't need full coverage - however, I don't think it is currently possible to exclude classes from testing once the JAR file is instrumented. There is a problem with Cobertura also in this area - that is why we have to copy the core-api and core-spi over in their "clean" version.