The ranking system may not have been explained in detail. When you level up, you get a Spartan Point, used to buy guns, AAs, grenades, and Perks. You need to reach Lv 50 to buy everything. At Lv 50 you can start a new specialization (starting as Wetwork or Operator) and they can get you gun camos (example Wetwork gets you DMR striped camo), armor, and most importantly the perk, unlocked at Lv 10 of your current spec. When starting a spec. It's easy in the beginning, but then takes longer to level up. BTW campaign legendary completed gets you MC armor.
Overall I think Halo is a great game.
I kinda had that claustrophobic feeling in the beginning of MM --- GT: Pointblank V