Quite frankly, I don't like many of the rules here. Now, I can still live with them (though I believe that when trying to create an atmosphere of roleplaying, slow expansion, etc., it must be up to the players, not constricted by rules), but there is one rule in particular that seems bad to me. Forcing a player to release a kingdom that he has fought for and has gained completely legitimately is completely unfair. It will mean little reward for crusades. I for one always like to create a Kingdom of Jerusalem, but there is no way I'm going to fight a war far away to simply give it away because of a rule. It takes away the sense of achievement if you're just doing something because a rule tells you that you have to do it. In my opinion, instituting this would ruin the whole concept of crusades in general, especially in Iberia and whatnot. If you want a game of slow expansion, yes, I do as well (though I believe some of the rules are far to restricting and that it should rather be the players setting the atmosphere by example). However, I think that forcing anyone to give up legitimate gains by use of a rule is unjustifiable.
That said, with the incredibly high amount of prestige required to claim anything, will that not only encourage more pagan-bashing? As Poland, if I have no way to expand into the Christian realms of Kiev, Hungary, Bohemia, or Germany, why not go on a rampage into the east and take over all the pagan lands? I think the high prestige cost, while a good idea, may be a bit extreme in practice. Perhaps Hard difficulty instead?