A fun, unique, and interesting game that allows you to immerse yourself in the role of a medieval noble.
The game relies on the dynastic system. So when you're character dies, you are put in control of the heir to his name, not necessarily the heir to all your old titles. So if you're not careful in positioning your heir in the path of power, you could find your powerful character dying and leaving you to play as a poor count.
You can expand your possessions through war or inheritance. Watch out who you go picking fights with, though, the AI in this game is a serious opponent, even on the easiest difficulty.
Crusader Kings is relatively simple for a Paradox game. The economy is pretty straightforward: tax the people and you'll get more money, but they won't like it. Trade is more or less non-existent. Diplomatic options are rather limited. You can have one ally, one lord, and some vassals. You can declare war, ask for peace, negotiate vassalage, and form your alliance.
Since this game is based on characters rather than countries, all diplomacy is carried on at a personal level. This can lead to some interesting situations. If the King of France likes you, the two of you can form an alliance. However, if either of you die, the alliance is dissolved, and your heirs might not get along so well...
You will often become attached to the characters in the game. You might have a particularly effective ruler who leads the country through a golden age of financial prosperity and expansion. Or you might get stuck with a crazy child king who lets the whole country go to pot while he blows his money on ponies. You'll often find yourself loving or hating or laughing at the games many characters and their amusing interactions.