Signing

Allow user to choose digest algo (from reasonable values).
One-button import of signatures into primary keyring.
Implement alternative ways to specify signing candidates (e.g., supply keyids in command line args or GUI and import from keyserver)
Better handling of case where no uids were selected to be signed.
Cache selected uids for subsequent runs in the event of error during signing or mailing.
Detect case where no uid with an email address has been selected. Possible handling: - Just warn that the signed uid(s) won't be emailed. - Force user to provide an email address to which to send. - Prohibit signing of this key.

Mailer

When no local mailer installed, determine OS and provide instructions for installing/configuring one.
Alternatively, implement support for detecting or asking for an alternative SMTP server.
Allow user to choose the From address for sending keys from the uids on the signing key(s).