If you're compiling a "support issues we're going to encounter" list, you might want to add this:

I was trying to create a test issue but found I could display the new issue form (either "normal" or pop-up variety) and complete it, but when I clicked Create, nothing happened. Must be a permissions issue, I figured so went around the admin pages until I was dizzy (still trying to get my head around the new security model!), but still the same problem.

Eventually I realised that the database I migrated contains a number of custom fields, some of which are required fields. Unfortunately, I had applied a field visibility scheme that hid one of the custom fields and, because it was required, validation failed but was unable to tell me about it because the field wasn't on the page. Can't add a default value for a custom field either, as they don't appear in the project default values page so if it's a field the issue creator shouldn't see, I guess you have to make it an optional field.