Defy already took care of everything except 3 and 5, so I'll explain it.
3. Nobody can see your kill death ratio anymore, not even you. The biggest thing that matters for ranking up is wins. I think it might be the only thing, I can't remember.
5. This isn't a game where you choose your loadout. Each player starts with the same gun, usually an assault rifle or a BR. If you see a gun that looks really huge, pick it up. The rocket and spartan laser are really good, but you find them on the ground in certain spots on the map.