I mainly ran with an Auto Rifle or Hand Cannon for my Primary slot and a Sniper for my Special slot. I love my Auto Rifle's firing rate, but the Hand Cannon also packs one helluva punch. When you find a Fallen Ghost in the field, it will unlock something called a Grimoire card for your profile. Not sure what these really do just yet, but they're similar to commendations in Reach. You can also unlock Grimoire cards for getting kills/ headshots/ etc.