Have you ever thought it odd that when night falls in Azeroth, the stars appear, the sky darkens, but the landscape itself remains as bright as it was in broad daylight? Once upon a time, the nights in WoW looked something like the above screenshot. I can't pinpoint the source since it was so many years ago, but the original reasoning was it changed so people who could only play during the night wouldn't have to always play in the dark. The current official line is that it was purely an art decision.
There have been innumerable threads on the official forums pleading for the return of dark nights. Blizzard has been listening and in patch 5.4 they added a "test case" for darker nights, but only in Stormwind, Orgrimmar, and Vale of Eternal Blossoms. The difference is there, but you need to compare before and after screenshots to really notice it. It still doesn't scream "night has fallen."