I have discovered that both Joomla! and Coppermine have an offline mode where everyone but the admin(s) is locked out and maintenance activity/changes aren't visible to people visiting your site. Only an offline message and admin login are there. This makes site design secure and easy, because you can tweak and play with the content and presentation without any surfers watching. It's also very good for those sites that you aren't going to be able to work with for a while. A good example is getting through the learning curve involved in setting up new software.
On several levels I find this a very useful feature. I would like to know if WordPress has this feature, or if there is any plugin that does this. In my opinion, this is a must-have feature, and if it isn't already a part of worpress (I'm new, so that very well could be the case), it should be a part of future versions. It would make plugins like "theme test drive" as wonderful as it is, unnecessary.