A list of good-to-know things about Crusader Kings
1) When your primary title changes, which is readily seen as your shield and potrait frame changes, you need to save and reload game.
2) Create Title button is only available if your personal demesne is part of any uncreated title. When it is visible you may create any titles that you and your vassals combinedly control enough of. Including titles you don't have any part of in direct control.
3) You can grant titles only if you have at least two provinces as your personal demesne, so you can't give any vassals duke titles if you have only one province as personal demesne.
4) Sometimes dead people become pope. This isn't a big problem though, because you can select the pope (so that his character screen comes up in middle of screen), press F12 and write "die" (without quote marks) in the console. Dead pope dies, new pope gets elected on dead one's place.
5) You cannot Usurp titles that don't have any provinces as part of them. You can Grab them though at higher prestige and badboy cost.
6) MP only: When you are a client, you will see rulers with many titles of their highest level being called duchy / kingdom of the alphabetically first title, while it's isn't their primary title. Example: Kingdom of Germany in 1066 is also kingdom of Burgundy and Italy. So clients will see kingdom of Germany called as kingdom of Burgundy, though shield is Germany's and the kingdom behaves in all respects as Germany. This is a cosmetic thing without any gameplay effects.
7) Sometimes you will see empty shields at some places. This is fixed with save-reload and it doesn't affect gameplay, except by making those countries hard to select for diplomatic actions.
8) If you create a new vassal when your country is in a war, the vassal will go independent after war is over. So create new vassals only during peace. You can grant titles to existing vassals during a war though. Also, if new vassal is result of courtier winning himself a province in a siege (see next), he won't go independent.
9) When you successfully siege a province held by pagans or muslims, the ownership of the province transfers to the siege's leader. You can see siege's leader in the small potrait in upper left corner in the siege screen (with the catapult and fort). Siege is led by highest ranking noble in the army.
10) When you click on a law in law selection screen, all other realms using that law will show up green in the screen. This is the easiest and best way to check inheritance laws in other lands. You can also see highest advances in each province at advance screen the same way.
11) When you offer marriage, only people to show up in first courtier screen (proposer) are single men and women with your surname (aka same dynasty) in your court. In the second courtier screen (proposed) it shows all single courtiers in the court you picked that are of opposite sex as proposer and are not brother / sister / mother / father or child of the proposer.
12) When you upgrade yourself from duke to king, it's likely that your count vassals liege remains your original duke title (which you hold too when you become king). So don't give your original duke title away, unless you are giving it to your last count vassal or you have no counts as vassals at all. This is corrected when your first king dies and his heir succeeds.
13) When you destroy a duke / king / emir / chief by taking his last province and free up his title(s) for creation, wait for a day after his destruction before creating any of the freed up titles so that liege-vassal relations clear up properly.
14) When you give a duke title to a vassal, all your current direct count vassals that are part of the land needed for that duke title will become vassals of the person you gave the duke title. This also applies to giving king titles to vassals (should you ever want to do so), but applies to both dukes and counts then. See "Listing of all duchies" FAQ to see what province belongs to which duchy.
15) Mongols can directly control whole map without demesne size penalties. Mongols treat everyone else as religious enemy, directly annexing their land without peaces. Mongols don't suffer attrition. Fear them.
16) Crusader Exceptations depend on your realm's (your demesne + your vassals) size and your prestige. The more you have, the higher it gets. Crusader Exceptations are reduced by having pagan or muslim provinces in your personal demesne. Crusader Exceptations affect only catholics.
17) You cannot give courtiers any higher title than count. You can only give duke and king titles to your vassals. This is because province ownership is decided by count titles and nobody can be duke or king without owning any provinces.
18) Sometimes you won't get random courtiers even if your court is too small and sometimes your new vassals won't get any random courtiers at all. This is fixed by save and reload, which fills all courts with random courtiers if they are too small.
19) AI plays by same rules as you. You can go through neutral religious enemy territory if you are at war with other member of that religion and AI can too. So, when you go on crusading, leave a significant home defense force behind.
If you think that something should be added to this list, don't post suggestion here, but go to the original thread (here) and post the suggestion there. If it's a good suggestion, it could very well be added into the list.