It uses Textile markup. This is a benefit, because textile is one of the various markup standards floating around the net, so it's not an particular idiosyncrasy of the wiki maintainer.
For OSX, the application bundle requires no separate installation of Ruby or Ruby on Rails. Distribution of the entire set is possible, but takes several megabytes. Upgrading an existing installation is easy: Enter the application folder, find the source directory, and replace it with the new source, restart the wiki.
Since Instiki contains its own webserver (usually reachable on http://localhost:2500/), it is inherently suitable for collaboration on your own servers or on your own workstation in a local area network (LAN). It will not work on some cheap webhosting account unless they have all these requirements installed.
Instiki has namespaces; it calls them webs, much like TWiki does. See WikiNameSpaces for more. Creation and administration of these webs happens just like the administration of the entire wiki.