Server can be a domain controller
Server can utilize more memory (4G as opposed to 2G for Pro IIRC)
Server can utilize more CPUs (4 as opposed to 2 for Pro IIRC)
Server supports more than 10 concurrent SMB clients
If you're trying to set up an enterprise server with DNS, you'll find it's great.
For somethings it's good, like Exchange. But for something as critical as DNS or DHCP, I'd prefer a unix box running BIND or dhcpd. They're much more reliable and much less maintenance.