Servers are designed to take large loads and not fail(hopefully) So they have things in place to help them have more uptime then a desktop. The Xserve has many of the same things as other servers by other companies.
Light out control. This allow you to run the system remotely we are not just talking VPN but reboots installs etc. Its a hardware level remote control. This allows someone from around the world to diagnose the servers software problems if it can't boot.
Redundant power supply. Power supplies are something that often fail, so there is two. The system can survive on one. If one fails the other one takes the load. Then you can pull the dead one out the back insert a new one and the users never even know.
Hot swappable drives. You can pull them out the front and insert a new one. So if your running a mirrored drive setup, one can fail you pull it out insert a new one and it rebuilds that drive...no one ever knows, except the accountants....when they get the bill.
It also has ECC ram...but the mac pro also has this. This type of ram checks for errors in what it is pulling from ram.
Normally servers also have more fans to help keep them cool. Also they are rack mount which allows you to stick way more machines in a smaller space. I think they can also be run Headless without a monitor.
I am almost certain what I have said is correct however I have never touched a Xserve only PC servers. But these things are the same and i checked apples site to conform they exist on the xserve. They may just be implemented differently, for example lights out. I don't know what software they use for access. I have only used HP Light out.