The Plot and Setting: The game takes place in the Forgotten Realms, and Tolkeinish world filled with elves and dwarves, good and evil, etc. The set up is that, long ago, Bhaal, the god of murder, walked the world and gave birth to hundreds of semi-immortal children. You play as one of his children, either embracing or resisting your demonic nature, as you adventure across the kingdom, searching for your destiny and hounding a mysterious wizard named Irenicus, who has his own plans for you. No paragraph synopsis can do justice to the wonderful story (without spoiling anything, anyway). The plot is filled with twists and surprises at every turn.
The Combat: The combat is 2nd edition AD&D, and believe me, you will have to try in order to defeat your enemies. Other RPGs just involve clicking on the bad guys and watching as your avatar does all the work, but in BGII, you will definitely need a good sense of tactics and strategy.
Graphics: While certainly inferior to the recent 3D games like Morrowind, Dungeon Siege, and Neverwinter Nights, BGII has some of coolest graphics and spell effects of any 2D RPG.
Cons: Not much. If you are looking for a slash-and-hack, look elsewhere. Also, the D&D rules have a bit of a learning curve for those not familiar with them.
Overall, Baldur's Gate II is definitely the best PC RPG ever, and is, in my opinion, tied with Cronotrigger as the best RPG for any system.