You can conquer half the game world (the Christian half, at least) within the lifetime of your first character if you exploit marrying off your sons and daughters to heirs of all the kingdoms. In the cases of agnatic succession you can even remarry the same daughter to someone else after you off her husband and her son and inherit the title for yourself.
It's funny to start as a small time count somewhere and go inherit the HRE, France, Denmark, England, Scotland, and all the little kingdoms in Spain (aragon, navarra, etc) just by marrying into the families and murdering everyone until your grandson inherits, then murdering the grandson so that it passes back to you. The fact that the 2nd heir will marry matrilineally just because you invite him to court (and he comes because he hates his daddy due to the pretender thing) is a bit silly and infinitely exploitable. All you have to do is invite him over, marry him to a daughter, off his brother so that he's in line for the throne, wait for a grandson, off the current king, off the son, off the grandson - bam. Four assassination plots and just about any kingdom on the map is yours.
It's only fun once, though - after that it just gets silly. So I go back to playing "normally" and conquering the world the old fashioned way.