In halo you played because playing was fun. The reward was the experience. In destiny it's not about the means it's about the ends. We play because we like the game on some level, but the real motivation is the loot. If a trade system was implemented then we would quickly aquire all the gear that we want and need. Not to mention that the gear would be less special and thus not worth grinding for. And don't even get me started on how people would monetize it. Imagine people trading Gjallarhorn, Hawkmoon, and intellect discipline ToO gauntlets that have hand cannon and heavy weapon reloader for $70-$200.