Computers used as datacenter servers have usage patterns that differ substantially from those of desktop or laptop computers. We discuss four key differences in usage and their first-order implications for designing computers that are particularly well-suited as servers: data movement, thousands of transactions per second, program isolation, and measurement underpinnings. Maintaining high-bandwidth data movement requires coordinated design decisions throughout the memory system, instruction-issue system, and even instruction set. Serving thousands of transactions per second requires continuous attention to all sources of delay – causes of long-latency transactions. Unrelated programs running on shared hardware produce delay through undesired interference; isolating programs from one another needs further hardware help. And finally, when running datacenter servers as a business it is vital to be able to observe and hence decrease inefficiencies across dozens of layers of software and thousands of interacting servers. There are myriad open research problems related to these issues.