AbdulAlhazred
Legend
System matters. Its very hard to do this in D&D, for instance, but baked in to DW and BW. Its doable to greater or lesser extents in other systems.
I know next to nothing of BW, but DW certainly is OK with a certain amount of DM prep. It recommends that the DM create what are called 'fronts', which are basically organizations and general plot arcs, possibly down to the level of some NPCs and a description of what these groups do, where they exist, etc. Remembering that in DW maps are supposed to 'have a lot of white space' and be basically just some names and a general list of ideas.
DW is also pretty thematically tight, the world is a pretty D&D-esque fantasy world, and the PCs are larger-than-life figures which populate it (though they may be reasonably limited in the capabilities at level 1, they are still fairly powerful to start with and don't have to 'build out' to their full potentiality in their class, a level 1 wizard is still a wizard, even if he's not a super powerful one).
You could run DW without any prep at all, it isn't exactly hard to do that, but the game as-written doesn't imagine you will go to that extreme.