I think it's pretty silly that you can't lay down. Pretty much the only shooter that you can't.
Shouldn't need to explain the benefits of being able to lay down.
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As you can see, people associate prone with that kids game CoD. Fair enough. Destiny isn't about sitting in one spot, though. When I played the beta, there were very few people camping in one spot, and they were easily killed. The game is about mobility, and it keeps it fast paced. Honestly, going prone is the opposite of that, and I'm glad it's not in the game.