OSX Lion Server Security
OSX Lion isn't an app as such but a collection of additional services and applications running on your Mac. Accessibility from outside is the same as for a plain vanilla OSX installation.

Any port you open allows access from outside, but access is always (as long as there are no security bugs) restricted to the service provided. So if you enable Web Sharing, access to the local web server is possible (including ~user/Sites) but not (unless you reconfigured the web server in an insane way) to your whole filesystem.