Savage biome areas support giant versions of creatures, as well as animal people. Civs also have varying degrees of settling in savage terrain, so it can be used as a means to control spread of civs. Lastly, embarks with savage biomes supports two groups of critters at the same time on each level (surface, and each cavern), while normal and calm only supports one.
There is no connection to night creatures, as far as I know, nor are there any savage specific underground creatures and plants (Highwood and whip vine are found in savage biomes only).
Humans have very poor tolerance for savage biomes and seem to need to tame the biomes down to neutral to be able to settle there. Elves have a higher level of savagery tolerance for forest terrain and can settle and expand in mildly savage biomes (something like a savagery below 70 or so, i.e. about 66-70). Curiously, I've recently observed dwarves that spawn on 100% savage mountain terrain, but they have a hard time expanding, although they seem to be able to do that as long as it's mountains. Goblins normally seem to have great difficulties starting on savage terrain and have a poor ability to expand on savage terrain. However, when modded to start only on glaciers they can start on savage terrain without problems, but have about normal difficulties expanding.
Mineral scarcity probably affect everything that's embedded in geological layers, but not the layers themselves. Some flux minerals are layer building, and thus are probably not affected, while others, all metal ores, and all gems are embedded in layers, and thus probably affected by the scarcity value. However, as far as I know there isn't any data broken down to lower level than overall correlation between embedded "stuff" and mineral scarcity in terms of amounts and variety (an image can be found on the wiki).
Megabeast and semi megabeast setting affect their numbers only. As far as I know they rampage with the same ferocity regardless of the savagery level.
By the way, have you heard about the thing called "upper case letters"? Use of those make it easier to read what you've written, and makes it less likely that people ignoring it because if it wasn't important enough to make it readable, it's probably not worth the effort trying to read it.