http://youtu.be/OmKlpIRPwo8 Agreed. A single guy could take over a whole keep. Or a few guys. NPC are necessary.
As for collisions, they would be nice in PvP. But I'm assuring they didn't put it in for performance reasons. In order for it to work properly all clients must have exact locations of everybody. In a MMO generally the exact and up to date location doesn't matter so much because abilities are still target based even if you don't use TAB in this game. This is why for instance when somebody fires off a range attack, and you hide behind a wall you still get hit. Not saying it can't be done, just that it's not always as easy as you'd think.
Also, not a big deal with NPCs since they don't move around so much, but players move way too much.