Aside from the fact that it's free, has much cooler pre-built site templates than most blogging services (including its sister company, Typepad) and it's incredibly easy to use, the thing that excites most people about the service is the degree to which you can control who sees the stuff you put on your blog:
Putting material online used to mean putting it up for anyone to see, search, criticize, record, or repurpose. Not anymore. With Vox, you can choose the privacy level for every post, every picture, every sound clip, every video. Put up posts for the world. Put up posts for just your family. Or just your friends. You can control everything. Or simply set a preference once and never worry about it again.
What does this have to do with nonprofits? By now regular readers of this blog know that I'm a big fan of blogging for personal knowledge management and learning. If I had my way, every staff person would have his/her own blog, if only as a repository for ideas, experiences, resources, etc. The cool thing about Vox is that it gives users a level of control that is really unheard of in other blogging services. You can literally decide which posts, videos, pictures, etc. can be seen and commented on, which in many ways makes it a much more useful tool. Staff could have some posts that were visible to their co-workers, others that were available to their boss and others for clients. And everything could be in a single location.
Personally if I was starting with a blog for the first time and wanted to see what all the buzz was about, I'd probably start with Vox.
On a somewhat unrelated point--for WordPress 2.1.1 users, this article on the need to upgrade is a must-read.