USB vs Ethernet cable length
According to Wikipedia:


  USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns. The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (192,000 km/s, which is close to the maximum achievable transmission speed for standard copper wire).


So with a delay per cable of 26ns and the spec requiring cable delay to be less than 5.2ns/m, that gives a theoretical maximum cable length of 26ns/(5.2ns/m) = 5m.

That source also mentions that USB 2.0 is limited to 5m, but USB 3.0 is not.