The first time I attempted to import i was getting
ImportIssueBean@101972a[<null>,<null>,<null>]
Obviously because I had forgotten to map summary and issue type. There is a third field which I am missing too.
Thankfully I write Java for a living so I can decipher this error message, but I can see no reason why a more meaningful message can not be provided to the user to indicate that mandatory fields are not being specified.

[23:39:01] ---------------------------
[23:39:01] Importing of Issues started
[23:39:01] Only new Issues will be imported
[23:39:01] Importing issue number 1 : ImportIssueBean@101972a[D01,1,<null>]
[23:39:01] FAILED: Error importing issue ImportIssueBean@101972a[D01,1,<null>]
[23:39:01] com.atlassian.jira.external.ExternalException: Unable to create issue: ImportIssueBean@101972a[D01,1,<null>]
at com.atlassian.jira.external.ExternalUtils.createIssue(ExternalUtils.java:268)
at com.atlassian.jira.imports.importer.impl.DefaultJiraDataImporter.importIssues(DefaultJiraDataImporter.java:379)
at com.atlassian.jira.imports.importer.impl.DefaultJiraDataImporter.doImport(DefaultJiraDataImporter.java:86)
at com.atlassian.jira.imports.importer.impl.ImporterThread.run(ImporterThread.java:16)
Caused by: java.lang.IllegalArgumentException
at com.atlassian.jira.workflow.AbstractJiraWorkflow.getLinkedStep(AbstractJiraWorkflow.java:169)
at com.atlassian.jira.external.ExternalUtils.setCurrentWorkflowStep(ExternalUtils.java:286)
at com.atlassian.jira.external.ExternalUtils.createIssue(ExternalUtils.java:262)
... 3 more