That's the narrative, but it never feels like it. Standing in the tower, you would never know it's in danger or under seige. The world does not react to anything you do.

This. I go to the tower and have no fear that it might fall at any second...everyone is either cheerful or rambling about something. There is no sense of urgency, if I wanted to I could just sit in the tower all day playing with my purple ball, and the Speaker, Ikora and Shaxx wouldn't give two rips. We need horde mode...on the tower. Fallen climbing the walls, balls of fire flying in, and a true sense that we're being wiped out. Now, don't get me wrong. I'm not bored, I've set my own goals (3 level 30 toons with the perfect balance of intellect / discipline / strength for my playstyle, maxed out weapons on all 3 so I no longer have to swap any gear). This will take me some time and keeps me hungry for more Destiny. And with the DLC right around the corner, there will be more weapons and armor to strive for. But to the OP's original point, there is a severe lack of urgency as to WHY I go back into the wilderness. And there definitely is NO sense that we're on the verge of extinction...