All Noble quests start with a minimum reputation of 10. This means that until you do some sucking up, they probably won't help you very much. Nobles like to smuggle (this puts you in conflict with the Thieves Guild), so be careful with those quests.
"If you are available for hire, I need someone reliable to carry out a certain, covert mission..." (rep 10 or more): take the emerald to the first contact, then take the item to the second contact, then either give the item to the Thieves Guild or fight four or five thugs (and take a hit on your rep with the Guild).
"If you want work, I am looking for someone to take a special item to an acquaintance of mine..." (rep 10 or more): another smuggling operation and (again) you have the choice of turning the item over to the Thieves Guild thugs or delivering it (and taking a hit on your rep). You get a ruby for your efforts.
"I need someone to bring an item to a contact of mine in <town>..." (rep 10 or more): take the gold to the contact and bring the item back to the questgiver. You may or may not run into a thug who wants to relieve you of the gold.
"The whining peasants who work my land near <dungoen> have long complained about evil wizardry being practiced in that dungeon..." (rep 10 or more): a standard dungeon crawl. You should see a slight boost to your rep with the commoners after completing this one, too.
"You may be aware that I am forming an alliance with <name>..." (rep 10 or more): a standard dungeon crawl. The dialogue indicates that a Giant is your target, but it could be anything. The only odd twist is that you must report to a different noble to collect your reward.
"I have an item that's very precious to me. It's been handed down from generation to generation in my family..." (rep 10 or more): the item was stolen by some orcs (good luck finding any orcs) and your job it to retrieve it. A standard dungeon crawl.
"If you could, I am in need of assistance from someone I know isn't a complete buffoon..." (rep 10 or more): you are to steal an item from a temple, get it to a contact in another location, then return to the questgiver for your reward. Your rep with that temple will take a hit if you are successful.
"Yes, I do have something for an able-bodied young <race>. It's nothing special...." (rep 10 or more): a standard dungeon crawl, but you'll get to fight a few thugs after you get the item from your contact.
"If you are interested in earning <number> gold for doing nothing more than delivering a little parcel..." or "I could use the services of someone of moderate talent..." (rep 10 or more): a smuggling operation. Deliver the item to the contact for your reward.
"You are not by any chance free to run a little errand for me?..." (rep 10 or more): another smuggling operation (why do kings have to smuggle? Aren't they the ones that make the import/export laws?)
"Yes, this is a potentially awkward situation I'm in..." (rep 10 or more): another smuggling operation.
"I have a mission of some diplomatic importance. A gift of peace that needs be exchanged..." (rep 10 or more): a simple delivery. Watch the time.
"Are you by any chance available for a little assignment that should not take you any time at all..." (rep 10 or more): a simple delivery.
"I have a somewhat complicated mission that needs a fourth man involved..." (rep 10 or more): a two-step delivery. Take the diamond to the first contact and get the second item, then deliver that to the second contact who will pay you.
"I was wondering if you might do me a small favor..." (rep 10 or more): another 2-step deliver; moving drugs this time.
"Are you free for a few days to do me a little favor? I just need something I borrowed brought back to its owner..." (rep 10 or more): another 2-step smuffling operation (there's a gem hidden inside the first item). The Thieves Guild is supposed to show up to collect the gem (if you don't turn it over to them, your rep will take a hit). Usually there will be four thugs for you to deal with.
"How would you like to have a lovely reward from my treasury..." (rep 10 or more): pick up the ingredient and deliver it to the alchemist.
"My cousin has appealed to me for aid in a rather distasteful situation..." (rep 10 or more): stop the consecration of an orcish temple by killing the shaman. A standard dungeon crawl.
"I have an acquaintance, an alchemist of some skill by the name of <name>..." (rep more than 10): deliver the Daedra Heart and fight the Daedra that come to take it from you. Tough for low and mid-level characters.
"If you have a couple days to spare and wouldn't mind being <number> gold pieces richer, I have a mission requiring some speed and delicacy..." (rep more than 10): a two-step delivery. Take the telescope to the first contact and pick up the item, then deliver the item to the second contact.
"I'm in rather a delicate situation... If I could count on your diligence and your silence..." (rep more than 10): retrieve a ring from a pawnbroker. You get to fight a lot of thugs afterwards.
"I was wondering if you might do me a small favor. You see, I have a friend who requires a certain shipment of a certain substance brought by a certain time..." (rep more than 10 -- there are three of these quests that are identical): another 2-step delivery.
"I have a friend, a potential ally, who is in need of discreet assistance. It's a relatively simple salvage operation..." (rep 20 or more): a standard dungeon crawl with a sapphire as your object.
"If you are available for hire, I am looking for someone to purvey a special item to an acquaintance of mine..." (rep 20 or more): sounds like a simple delivery, but you'll be attacked by orcs (a lot of them). One of them will give the name of a contact. If you go talk to the contact, you will find out that the item was stolen from Orsinium. Giving the item to the contact will boost your rep with Gortwog (unless Gortwog gave you this quest in the first place - the game's a little flakey in this respect).
"I need someone to bring a certain item to a contact of mine in <location>. You will receive an item in return..." (rep 30 or more): a two-step delivery that will probably put you afoul of the Thieves Guild. The item will be stolen from you, but you can track it down and retrieve it (you'll find the location on your attacker's body). You will also get a magic item as part of your reward and the location of a Witch Coven.
"I am in trouble with a group of daedra and am looking for someone to purvey a rare manuscript to an acquaintance of mine..." (rep between 40 and 49): deliver the book and fight the Daedra on the way. A very tough quest for low and mid-level characters. Turning over the book will blow the quest, but you'll gain a few brownie points with the powers of Oblivion. Completing the quest should net you a map to a Witch Coven (unless the location is already on your map).
"...always a pleasure to see you, my dear friend, especially when I need help..." (rep 60 or more): deliver an item that the local vampire tribe wants. You can turn it over to the vampires and gain a few points (plus some gold and a magic item) with them. Delivering the item (and killing the vampires that try to take it from you) will net you a map to a Witch Coven (unless the location is already on your map).