Probably the best RPG in years to come. A foundation of what is in the future.
Good thing: The dungeons and city's are incredibly atmospheric with a live and creepy feeling where it's wanted. This is backed up by the game's awesome graphics.
Good thing: Oblivion sports beautiful graphics. Your armor and items look's and shines well. Character's look realistic with the facing and expressions. I recommend going onto the highest points in Oblivion then tuning up the graphics just too look at the breathtaking view of Cyrodiil.
Good thing: The boring combat system from Morrowind is now dunked for the new dynamic system. Now shadows do matter so you wont' stand in broad daylight and the enemy for some reason can't see you. There's more of a reason to use stealth at night. Now you can block with the click of the mouse. Special Power attacks help disarm and incapitate your opponents. Snuck up on him did ya? Hold down your attack then let loose a critical hit which'll probably kill him instantly.
Good thing: The new AI system really makes the game feel alive. It's not the same NPC in the same spot for 2 weeks. NPC's eat, drink, read, work, talk, and pray. Oblivion feels like a seperate world.
Good thing: There's a very good reason why the game is so great and complex without being confusing. It's got a great interface. The journal is updated and helps you keep track of where you want and need to go with trackers and pointers.
Good thing: The sound in the game is top notch. Voice acting is superb especially main characters in the Main Quest. There's something unique from every NPC. Your sword clangs to the shield or wall. Your footsteps change when you walk on stone tablets or forests.
Good thing: Freeroam aspect is great. You can explore all of Cyrodiil without any quests or agenda just to kill bandits animals and such or explore.
Good thing: There's alot of classes and skills to learn. With higher skill levels bring new tactics in combat. Wanna beat that tough boss easily? Buy ingridients then sport potions that ubersize your stats.
Good thing: The RPG newbie interface is friendly and accessible. It's easy for RPG newbies and not annoying to RPG veterans.
Good thing: There's a very creative list of monsters and all characters are dressed appropriately for the occasion. Example: Sleeping guards are out of uniform armor and into normal clothing.
Bad things: All good things come with faults. The graphics are great but they cut into performance and increase load times. You need a beast of a PC to enjoy the game at max strength.
Bad things: All freeroam games have flaws in AI. Sometimes the game's AI may just break. and stand there taking your blows.
Bad things: Some of the dungeons are reused and may feel the same with the bland textures and such.