Paradox often releases games in a fairly raw state, basically at a beta level, depending on input from their community of fans to rapidly iterate and polish the game post-release, with multiple patches. Crusader Kings II is, however, quite polished and playable out of the box (though there have been several patches since its release) -- doubtless, the fact that it's a sequel helps (although it didn't help Victoria 2, which was a bit of a mess when first published).
As with most of Paradox's games, Crusader Kings is a grand strategy game spanning a long stretch of history -- from 1066 through the 15th century. The game engine is based on Europa Universalis, but the gameplay is quite different, because the concerns of Medieval rulers were quite different from those of Early Modern ones.
While warfare is certainly important in the game, as with Paradox's other titles, you cannot simply embark on world conquest; if you do, you will mightily piss off the rest of Europe, and likely find yourself at war with powerful enemies, excommunicated by the pope, and with your own vassals in revolt. Instead, you spend a fair bit of time intriguing to obtain claims to demesnes you don't control at present, as it's very hard to hold one without a claim; intriguing to gain the favor of your liege in the hope that he will grant you new titles; and so on.
Additionally, the number of demesnes you may control directly is limited by your rank in the nobility; beyond a certain point, you must grant titles in your realm to vassals, and then work to keep on good terms with them so they'll support you in the event of war -- and won't rebel, of course.
There are a huge number of characters across Europe, each with their own stats and characteristics; your own court holds several, and your advisors are drawn from members of your court. A strong advisor can have a big positive impact on your realm.
The critical thing for long-term success, however, is dynastic management. Marriage alliances are often essential for military success, and can, with luck, open up claims on other titles; but more than this, the stats and characteristics of your heirs are partly dependent on those of their parents. And the monarch's stats and characteristics are critical; if you have a cruel, heretical kin-slayer as monarch, with low values in essential stats, you may find your realm disintegrating as vassals revolt and nearby powers invade. Consequently, you want to find a mate with good stats and characteristics; ensuring "good bloodstock" among your line is among your most important considerations.
This is, in fact, one area in which Crusader Kings II is a great improvement on the earlier title; in the original Crusader Kings, when looking for a mate, you had to click on one demesne after another, browsing the available marriageable women there, and trying to find one with excellent stats. It was like searching for a needle in a haystack, and quite dull; the result was that you often settled for mediocrity, which made game success that much more of a struggle.
Of course, a fan of the game built a little tool to scan data for all characters in the game and rank marriageable women by whatever criteria you chose; this solved the problem of tedium, but made the game almost too easy. Crusader Kings II provides an appropriate middle ground; you can call up a window listing a bunch of potential mates, giving you a reasonable selection, but not access to everyone in Christendom. This strikes me as the right point of balance.
As with other Paradox titles, Crusader Kings is quite complex, and has a steep learning curve; it is not for those who eschew manuals entirely. But it is an amazingly rich, detailed, and evocative game that strategy gamers willing to tolerate complexity will doubtless adore.