Ryuk explains it clearly. There are 2 main differences related to the death note between humans and shinigami. The first is that shinigami can take a human's remaining life force and add it to their own. The second is that shinigami eyes can see name and lifespan.
Rem also explains clearly that the only way to give a human additional life is if a ...
L did have emotions but Light not.
L really considered Light as a friend, but at the same time was sure that Light was Kira. We can see that when L was dying (ep. 25), L was regretting that he is sure that Light is Kira, but has no way to prove it. So he still showed gratitude to Kira, and wiped his feet.