Hi!
I think there have been a few threads where people tried to collect information on the parameters. In the end, we are all trying more or less blindly, learning from our mistakes.
As for sheer cliffs, as far as I know, you can't have them right now. Any cliff wall in the game will currently be equipped with ramps.
There are a few hints for world gen I can give you:
* If you want a world with a long history, nullify the percentage for megabeast death. It is extremely difficult to find a world where megabeasts live longer than 600 years, so if you want to see the year 1050 in world gen, you have to forget about living megabeasts.
* At the very end of the parameters list, there are minima for low volcanism, mid volcanism, low savagery, mid savagery, etc. Unless you really, really need one of these parameters (and you will probably know it when it is the case), these are just unnecessary rejects. Therefore, nullify them. The only time they can be very useful is if you wish to create a world with a lot of civs (SMALL with 60 civs, MEDIUM with 100 civs). In that case, set low and mid savagery to a value similar to your minima for forest or some other major land type. Civs can't be genned on high savagery, but they are the last thing the generator creates, so it wastes a lot of time only to notice that it can't place all the civs. By using the minima for savagery, you can make it check much earlier, speeding things up.
* The variance values determine how large the forests, deserts, etc. will be. Roughly speaking, 50 - 200 will give you relatively large, homogenous regions; 400 - 500 will give you what I would call a medium variance; 800 - 1600 will give you radical variance with 1600 kind of making a really spotty map. Note that volcanism does not seem to influence terrain, so this variance can be handled freely. Also note that variance values are adapted to map size in a rather illogical manner - the smaller the map, the smaller the variance, meaning that smaller world will have even less variety than a similarly sized portion of a bigger world.
* Unless you wish to experiment with large regions, set the maximum number of regions to 5000 (which is the max value). Otherwise, you may get rejects just because of too many regions.
* Civ number is relatively conservative, so you can increase it without much problems. I also suggest you nullify the population cap (unless you want to experiment with limited civs).
* In MEDIUM worlds, you can usually have all 800 rivers, if you like.
* Caves are usually filled with giants (and maybe cyclopses and ettins); they hamper nearby civs and can even destroy them (especially elves). Still, the default settings surrounding caves are very small and you may wish to experiment with increasing them a bit.
* Good and evil settings are very flexible (there seems to be no conditions for placing either in a tile, except for the absence of the other), so you can experiment with them without too many rejects (unless you make the values too large making them sum up to more than your map size :) ). In my experience, you can increase them up to 4 times the default without any negative effect.
* Volcanoes are cheap, you can increase their number and as long as you have at least medium volcanism variance, you will usually not see many rejects because of volcanoes.
* Peaks, on the other hand, do require specific height parameters, and unless you make a mainly mountaineous world, you need to be careful about increasing their number.
Well, these are about the tips I can give. Have fun!
Deathworks