The lack of diplomatic options frustrates me. For example, if I win a hard fought defensive war against a neighboring kingdom (who used a fake claim as their CB), I certainly want more than a small amount of gold as reparation. You should be able to force them to cede a few provinces of your choice, preferably those that directly border your territory. Or exact regular tribute. Or take hostages. Or all of the above if that's how you want to do it. At the least, you should get a county claim CB on them for a few years.
In regards to the CB system, as in not being able to launch a war without one, I think it's a heavy handed way to go about balancing the game. The possible repercussions of attacking without 'cause' should be considered and used as a balancing tool instead of outright blocking you from war without a CB. With previous Paradox titles (EU and Rome spring to mind), you could declare war without a 'valid' CB, however there were various negative penalties associated with doing this, like dropping stability, lowering opinion and the perception of your kingdom/empire in the eyes of other rulers, so on.
My personal belief is that the excessive use of CB is to try to slow how much the player could actually do.
Since a looser system would show that you could quite easily take over all of Europe.
CB are essentially the game's speed bumps to make sure you aren't playing too fast.
I'm leaning towards this in addition to the fact that CK, unlike other PDOX titles, is focused on family intrigue and plotting. There are better ways to go about slowing down the game in regards to conquest though. The old penalty system is one of them. Augment it with slightly more tangible penalties like, say, province modifiers on land that you conquer without a valid CB, set so that they are MUCH more likely to be the source of common revolts/uprisings for many years to come. Gain too much land via 'unjust' conquest too quickly and you'll be beset by angry locals. Or something along those lines. I'd prefer almost anything to the current clumsy stop gap.