Exposing a web server incurs increased risk. If there is a "zero day" exploit, nothing but an internal network that's not connected to the Internet at large will protect you.
That said, Windows Home Server is built on Windows Server 2003, so it has that operating system's built-in firewall. Further, Windows Home Server only exposes the minimum set of ports to the internet: 80, 443, and 4125. So you're relatively safe. If you make your server available on the Internet, you should anticipate that it will be probed fairly regularly; mine is, at least. (Servers on home networks are a prime target for hackers, spammers, botnets, etc.) However, I've never had an intrusion.
I'm not on the WHS team, I just post a lot. :)