A few points that would make this really annoying.
The AI does work like a country, plus it fights the player most. Means that the will claim your land although you gave them theirs.
There is no relation system. That's really a problem, as we can't say 'I give you some land and then we help each other'. The game makes no difference between relatives, and you have no control over non-vassals. Your influence on your neighbour in CK is like none. You can fight them, yes. You can make alliances. But that's it. You can't warn them, you can't tell them that you gave them land and can take it back too. There the system is really weak, and we can only hope for a better version with relations. But that won't be possible for CK and has to be part of CK2 I guess.
Besides, the gavelkind doesn't really work. If I give a son a Kingdom title, he will have it but I will keep the vassals. And he will also get titles that are not in the same area. What do I do with a King of Jerusalem that has lands in Saxony etc.? There it would be better you are forced to choose between your titles and then give some up. Like there is a point you have to give up a Kingdom title and some Duchies and you can choose personally which you give up. Then you would give him the lands in Jerusalem and keep your own land free of them.
And what I really would like to see is that you can give up vassals to one. Not only the titles, but also the vassals you have. I mean you say I don't like this vassal in my realm, but he is not part of the title I give away. Then there should be a possibility to do that. Like in my AAR: I have given away Jerusalem to my son, but Aleppo is still part of my realm because it is part of Syria and I don't have the title. I can't get it either, since the Seljuks own extremly much land. So this vassal no stays under me and not my son, although I don't want him to be part of my realm. Also he doesn't declare independence, which means I have to wait years to get rid of him. If I may choose to give him away I wouldn't have this problem.