I'm sorry, I'm AM self-whoring now. :-)
I should also mention, and I think this is an important thing to list on all of the modules above, is support for message/error/label localization, both internally in the modules, or externally via callbacks or something.
In the case of Mango::Form, labels default LABEL_FIELDNAME and error messages default to FIELD_CONSTRAINTNAME. These can be overriden in the config above, but are all mostly passed through L18N like Maketext, or the Catalyst::Plugin::L10N or other such things.
Once people find a good form handler, then a good validator, the next things that usually bites is the localization of grouping of the messages themselves.