The short answer: ECC buffered RAM and dual processor support.
The more detailed answer: Server motherboards are designed from top to bottom to support a much greater workload from multiple users. Better board circuitry, faster bus speeds. That's not to say you have to use server hardware for a multi-user system. I recently built a server to support ~8-10 users, all moving files around constantly and running memory intense applications, using an Asus Rampage board, an i7-990x Extreme CPU and 24GB DDR3 RAM. It's never had so much as a hiccup in performance. However, be certain that the servers that run Facebook and Google are far more robust as they have to handle heavy traffic constantly.