Job description is under the Nobility heading:
"The Expedition Leader is the dwarf elected by the Founders to lead the expedition, or the dwarf with the most social skills among the Founders, or is a randomly selected dwarf among the Founders. The Expedition Leader is responsible for meeting with disgruntled dwarves, foreign diplomats, and is generally the guy to blame if things go bad. Once the Fortress Population reaches 15, this noble position is replaced by the Mayor."
Makes you the face of the fortress, basically. The position is necessary for interacting with foreign dignitaries and the Outpost Liason, and it would certainly be nice if you tried to talk down the crazies. You hold the position until you become Mayor automatically, which gives you expanded Mandate/Demand powers and a nicer room. You can be voted out of the Mayor position, however.
EDIT: Sneevil beat me to it, and has the right idea. Specifically, the skills for helping out mood is Pacifier for the angry types and Consoler for the sad types.
Broker is also a pretty important job that needs filling; not that the lack of one will stop trading, but developing ranks in Jude of Intent, Appraiser, and Negotiator will certainly make trading more profitable and faster. You can certainly get by without a sheriff, he's just the guy who is sanctioned to beat on other dwarves. Nobles are unhappy (read: crazier) if their mandates are ignored and nobody is punished for it, and dwarves are unhappy if they get beat on by other dwarves and nobody does anything about it. Certainly a good noble to have later on, but not necessary, strictly speaking. Bookkeepers are convenient, but can really be done by anybody who's bored for a week and don't require much maintenance.
Also, The Nefarious Spike doesn't fit the format of a Fortress Name. Fortresses tend to go more AdjectiveNoun, or NounNoun. However, I forgot about the fortress government completely, which is the format you followed. Since the gov doesn't have a name, I'll put it in for that.
Fortress Name Votes: Bomreklolok, Whippedgranite (1)
Government Name Votes: The Nefarious Spike (1)
Terrain Votes: 2 votes forest, 1 vote mountain, 1 vote ocean, 1 vote Low Evil, 1 vote arctic and 1 vote temperate