I beg to differ about the inability to change things "a bit." The other day I tried to regen a world, and for the convenience of not having to type in the seeds again (which is annoying since you can't use delete and have to start over for any mistake) I just added the world_gen.txt for that world as an option in the advanced-design-parameters screen.
At first it was messing up REALLY badly and just kept making and throwing out several worlds a second, getting caught on the "making rivers" part. So I checked and for some reason the world_gen.txt had an enormous amount of weird strings of numbers and crap, which I don't THINK is important or means anything, so I deleted that and repasted.
This should give me all the same terrain, civs, names of civs, creatures, historical events, moisture, volcanism etc., right?
Well, the lay of the land was certainly the same, and a lot of the same civilizations and stuff were present, but towns were different and some necromancer towers were in different spots, though close to their original spot.
Also, there was a vampire who was a lawgiver the first time around, but in the second go the history says her cover was blown (by another vampire, heh) about 100 years ago and is currently a farmer somewhere.
I actually made a thread about it, but most of the stuff in it I just put here too, and actually in greater detail. I'll link it in case anyone cares.
http://www.bay12forums.com/smf/index.php?topic=101911.0