@alexender00 - regarding focus vs. spread
I tried both, and now am switching from concentrated to spread - I was kinda forced to focused approach early in the game (Gavelkind and uber-horny ruler with 9 sons anyone?), and it seemed like a good idea, but I didn't concider few things:
Cities - they give huge income (and I don't even have max tax), which again proves to be the key to power.
Tech - At some point in my game I just stopped using steward and marshall for troops/income, and instead switched to tech improvements - it is much more efficient when you have bigger demesne (tech spreads much faster, and each province it spreads to hastens it in others - this gives more developed kingdom as a whole instead of uber 30/30/30 province surrounded by 13/13/13 ones). It also speeds up your vassals advancing. I know most prefer to keep their vassals as weak as possible, but I just feel different about that one ^^ (as in: I am against tipping the balance to the point, where vassals are so weak I stop benefiting from them).
Vassals - more counties in your hands = less counties in vassals hands = you are in better position overall. We are limited to 2 duchies, but who said you should create them? (this is especially true in baltics area: playing as fx. poland? Forget duchy of Pomeralia... Hold all the counties yourself, never create anything and forget the penalties).
Note - once going for spread, focus on coastal provinces - them cities are much much better with ports
.