2. stacktraces != debug. I don't actually see any difference with java stacktrace (only that it contains groovy classes, but it's still exactly same format). I mean tools for debug, like debug in IDE (look at Intellij IDEA, very powerful in debug), or JMX, take a look at VisualVM and profiling tools. There're many different tools. It's not just stacktrace.
5. Agree, not so popular that plain Spring maybe. But still, we're talking about features, right? not amount of projects on prod. Grails is pretty comparable to Rails/Django, in terms of features. Also, version 3 is too new, it requires to rewrite a lot of stuff to migrate. It takes time.
5. Grails adoption is tumbling. Not many places are starting new projects in Grails, or even upgrading from 2.x to 3.