While the graphics are slightly outdated, they were very well done for their time. And when combined with the history behind the game and its wonderful soundtrack, the game comes across as extremely polished.
The large scale of the game controlling fleets in may solar systems allows for a leisurely pace to play, which makes for worthwhile way of controlling ones ships across multiple places. This makes for less stress in the game due to how intuitive and evenly paced the game is.
If one player decides to fight in real-time instead of auto-resolving a battle, the other player joins the fight on their team. The owner of the army can then delegate control of some of their units to their teammate. This system brings a lot of fun and strategy into the game.
The game allows for one to make their own choices with there being many "win" conditions in that you can choose to progress how you like. Want to take over all of Europe, that is possible, want to create a successful lineage of family that keeps to themselves, this is also possible.
Crusader King II is a very complicated game that seems overly complex at first and may take a while to really learn as there are quite a lot of things to not only balance in the game but also figure out how to do either through in game tutorials or reading online.