Hmm, well these are just me being picky.. and I think you already plan on doing some of them, but just incase..
- Show users avatar when they comment in the gustbook.. (if someone doesn't want it they can remove it from the template.. so you don't have to add a "yes/no" option in the admincp)
- Guestbook is on by default.
- Guestbook HTML is a template rather than hardcoded (this isn't a big deal, just makes it easier to skin

)
That's about all I can think of. Maybe allow HTML in the guestbook (basic img, b, i, etc) or allow guestbook attachments that create thumbnails, like vb does. That's not really all that wanted for me though.