"* First of all there's the tickless kernel
The tickless mode is an option in the newest kernels keeping power
consumption low by having the processor staying asleep, in the
normal state the kernel is waking up the processor on an automated
interval. With the tickless kernel it's possible to wake the
processor only when there's a real need for it. Effectively saving
on power consumption in notebooks and servers alike.
"* Not forced to use anything you don't need
Linux gives you the opportunity to take everything out you don't
need, this makes it much more efficient. I can't imagine running a
dedicated webserver with a gui. I don't have a clue how someone can
come up with the idea of doing this. A Gui eats resources that's
obvious and when you are using windows to download a big file at
home, when your asleep it's still fully running. I download files
while asleep on a minimal resources system."