Basically all that's been done is add more of the same. Which with Oblivion is more a compliment than a complaint.
The Shivering Isles and The Nights Of The Nine are included in the game of the year version. Basically its the same stuff just more of it like getting another 100 quests or so. It's worth it to pay the extra couple bucks.
The Shivering Isles main quest alone can add up to 20 hours and there is plenty of side missions and exploring that could easily make that number much higher. There are plenty of enchanted items in The Shivering Isles and the landscape is bizarre but wonderful. There are some new things added a few summons, creatures, clothing, armor, and weapons. In this DLC you aid the Mad God Sheogorath you save his land from the greymarch a devastating event that happens once a milennia that destroys all the Isles. All the inhabitants are seriously demented and you'll find yourself doing some crazy but fun things. You can work your way up to a Duke and beyond his court of madness. Theres not many unique items, but the situation in which everything is presented is surely something new and the map that comes with this realm is a third of the size of the original one at least so theres plenty of places to go. Unfortunately your actions in The Isles doesn'y really affect your standing in Cyrodill.
In The Nights of The Nine you recieve a bunch of fetch quests, fun nonetheless. Basically you have to find the weapons and armor of the lost crusader to finally kill a daedric horror that has come to finish off Tamriel. You recieve all these unique armor and weapons and some are quite good so you'll enjoy having them and in the end you fight a cool boss. This expansion isn't very long probably a couple hours.
The graphics were nice really impressive, especially for that time, which is only a couple years ago, but games are constantly improving. Everything is crisp, and beautiful, the lighting is fine, and most everything is in great detail. The only problems you may have with the graphics is the occasional body part exposed through a wall or such whic is to be expected at some point in all games.
The gameplay is amazing. It's almost to a point where the sheer massiveness of freedom concerning gameplay should make you call it more than a game, because all too often its like a second life. The way Oblivion works, is you can develop a character anyway you want them appearance, race, attributes, skills, etc. You start by making the character appear anyway you want them from anything from an Argonian Lizard Man to an Orc. You can then customize how you look from there, the possibilities are truly endless. Its basically a giant role playing game. After you choose your more cosmetic attributes you can choose seven major skills from a total of twenty one. This seven major skills work in this way you level up by increasing them so you can level up by shoping at your favorite merchant or pickpocketing people, or maybe casting your favorite shield spell when your not even involved in combat. Thats the good thing about Oblivion you don't get a set amount of experience points for vanquishing foes and then put a couple points to some general attributes you level up your specific skills and then once you increase any of your skills or a total of them by ten you level up and can upgrade the more general basic attributes commonly seen in games Strength, Endurance, Luck, and the like. Once you get to a certain level in a skill you recieve perks like being able to cast higher level spells or new sword moves or maybe even better deals when bartering.
There are also a number of factions to join on this game so you can be a Gladiator, Assasson, Thief, Mercenary, and more. The main quest involves you finding the lost heir of the empire who is the only one who can close the gates to the realm of Oblivion which is filled with demons and other terrors. You find that a cult is behind the opening of these gates and you work with the Emperor's personal guard to stop them if you wish, or you can run around and become the Archmage or slaughter everyone or maybe even do anything and everything across the many small hamlets spread around the map. And the map is huge it's massive and no part is truly empty you could wander it for days and not find even half of all the locations. There are hundreds of different characters in this game and they all sound great. The music is good as well fits the tone of gameplay and can be truly epic.
In the end Oblivion is truly a masterpiece and whether your a fan of the genre or not you should buy it.