A few further points,
In terms of categorisation, I agree with that tags tend to be a better solution
Replying to an email address to subscribe is a really nice solution. It doesn't require the user to visit a separate website, it doesn't require them to manually create an account or add their email address. These are minor things, but will significantly increase the number of members who decide to come over to the new group. I would also suggest that it might be worthwhile to temporarily make directly adding a free feature in order to entice groups to migrate over.
Also, it might be worthwhile seeing how your competitors handle migration. I performed a search for "migrate from google groups". Groupspaces allows you to import a .csv from your group (http://help.groupspaces.com/customer/portal/articles/68464-migrate-from-google-groups-and-import-your-mailing-list). At least in 2009, John Resig found that the only way to export messages was to perform an IMAP dump from his email account or to screenscrape the site (http://ejohn.org/blog/google-groups-is-dead/).
@Shal: Why does each member need to authorise the transfer of their messages? What the difference between moving mailing list providers and moving web hosts, so long as the new host offers reasonable protection for users privacy? Certain groups may contain a higher degree of confidential content and this caution may be warranted, but these groups should accept that if someone wants to search the old messages that they will have to search the old group.