If I either change the passphrase or export a secret key, multiple 'set
passphrase' dialogs pop up - I was very confused by this, but with some
experimentation I found out that this is one dialog for the master key and one
for each subkey. The problem is that neither the dialog nor the command prompt
tells for which key the current dialog is. I guess most of the time people will
want the same passphrase for all subkeys and probably also the same for the
master key. If that is the case, it should not be necessary to enter the
passphrase more than twice. But even if they want different passphrases, the
dialog should tell which subkey is affected at the moment. I guess the best way
would be to use the same passphrase for all keys by default and allow changing
the passphrase for specific keys by adding their IDs to the passwd command.

For exporting secret keys, I guess if someone really wants to export different
subkeys with different passphrases it is always possible to export into
different files, so a good default would be to have one passphrase per file.
However, I would also like an option like in the old (1.x) versions, namely
"export in the same format that you are using for storage, without decrypting
the secret key and re-encrypting it again". That format would no longer be
compatible with other versions, but at the moment this is just a pain.

Another problem is that setting an empty passphrase when exporting is broken,
but I will open a separate issue for this.