When you add a choice or lookup to the CommonViewField for a ContentQueryWebPart, if you had ticked the allow multiple-values checkbox, the CQWP will return you nothing.
Couple of people blogged about this:
Adri Verlaan [MSFT] even says:
Unfortunatly the data source that the CQWP uses does not support Mutli-Valued Lookups or Multi-Choice columns.
Turns out after a bit more digging around, you _can_ use multi-value choices. To get multi-value choices to work, you need to use a different type:
Instead of Choice, use MultiChoice
<property name="CommonViewFields" type="string">MyDescription,PublishingHtml;MyTags,MultiChoice</property>
I have not had much luck with Multi-lookup and I suspect there might be some truth in what Adri was referring to. If I consider how SharePoint might have to work with a field that contains multiple keys to a different lookup list, this problem becomes far more complex.
Anyway, at least it works for choice.