We're going from most to least essential with this list, and you can tell from the order how much I love my spork. We all need something to sleep in, though.
The choice of a sleeping bag is another deeply personal choice, so consider your options. Note that men and women are differently exothermic, so a sleeping bag that is comfortable for your partner might not be comfortable for you. I like to think of the temperature rating on the bag as the temperature above which you will not die. You won't necessarily be comfy if you are a chilly sleeper. If you are a furnace when you sleep, you're freer to choose a lighter bag. Also if you never plan to cold-camp, take that into your decision. Are you a sprawler when you sleep? You might wake up feeling extremely confined in a mummy bag.