Firstly, great plugin. It's really elegant code and very easy to understand - something sadly very rare these days.
One thing I've noticed is that if you edit the subtitle using the quickedit, then it actually completely blanks the subtitle, rather than saving it. Can't see any obvious hooks as to where you've saved the edits from the quickedit bar, perhaps that's meant to be handled by WordPress?
Again, thanks for a cracking plugin and one which people should look out to see how to code 'properly'