Why do people keep misusing the word "genocide"? The burning of Teldrassil is not genocide. A terrible act of war? Sure. Mass murder? Sure. But no, it isn't genocide. To call it genocide would imply that this is a part of some systematic attempt to wipe out all Night Elves.
Additionally, nothing form datamined text so far even gives me the impression that Blizzard is aiming to portray an act of genocide. The tree is not the Night Elf race and we don't know how many Night Elves died in the attack anyways.
By the same token, I don't get the impression the Alliance are committing genocide against the Forsaken either, even though they are also losing their homes.
Citation?