You unlock the UNSC weapons at level 2, Covenant at level 5, and Promethean at 22. By level 22 you will have every Primary/Secondary/Grenade and some Armor/Tactical/Support unlocked. It doesn't take long at all, maybe four hours or so into the release I was above a 20.
BR used for close to mid, DMR for mid to long, and the Lightrifle at long range. Halo has always been fast paced, not to degree of CoD and it still isn't to that pace and if you think Battlefield is fast paced then you have no idea what you're talking about.