There have been plenty of MMOs with day/night cycles and weather but what I really like about GW2 is that the day/night and weather will have an impact on the world as affecting abilities, events.
Most MMOs I've played have had a day/night cycle and a few of them had a noticeable weather system. Even WoW had some of the craziest rain storms I've seen in a game. The nights in most MMOs haven't been that impressive because it never really feels…