All the credentials are imported; although, since we do not support non-OpenID accounts, those users would have to create an openID... but their credentials would transfer.
Questions, answers, comments all transfer.
The reputation and badges have minor differences between the sites so there will be a reputation recalc on all the accounts after they are imported. The final values don't tend to vary greatly but there may be differences. New badges may be awarded since we've added new badges to the system, but the majority of them will remain the same.