I'm loving all of the plugins the WPMU Dev has to offer, but I would love to see one more added to the list: Maintenance Mode.
When you're switching themes, it's really convenient to be able to allow only admins to see the WordPress site and show everyone else a, "Hey, we're busy, but we'll be back shortly!" message. That way people don't try to navigate your half-broken theme as it's being set up. What do you guys think?