Most server motherboards are multi-processor capable. This may not seem like much at the onset, but increased load may require more horsepower.
Many servers have redundant power supplies built in.
Most servers offer two NIC cards for multi-homing or redundancy. Dell, in particular, offers far better service on servers, including rapid-turnaround live tech support on the operating system of your choice. You don't get that with desktops. By the way, my SOHO network is built around a Dell 1300 server - more than adequate. I often use Dell desktops as file and printer servers at client offices, but only after explaining the few shortcomings.