Features like hotswappable and redundant power supplies are nice buzzwords to satisfy CIOs but in practice, they only add to the cost of the machine and are not 100% guaranteed to work.
You should have all of your important data and configuration on a SAN rather than storing it on the server which is backed up often. Storing anything other than applications on a server is lunacy.
Redundant power supplies do provide some chance of no service interruption but you really should have failover servers in place to take over the load when one server goes down rather than relying on redundant systems within a server.
If you are storing your critical information on the server then you are an idiot. Any significant hardware failure should mean that you take the box down, remove it from the server room and repair it. Repairing it in the rack is not practical even if things are "hotswap".
That said, Apple should reconsider offering an X-serve in the future. It is possible that they may bring back the line when lightpeak comes out.