Goblin towers, forest retreats and Dwarven halls do not exist yet. They are planned to be added in, most likely after the human towns in the upcoming version.
Enemy encounters seem to get larger if you have a large party, but their skills are just the same as you would find them alone or with a party of three. In short, they do not gain skill bonuses.
As Irenices already stated, metals have trouble piercing through the same type of metal, and inferior metals have even greater trouble piercing through superior metals. This technically applies to a material's stats, and as such, if you were to mod skin to be as tough as steel, copper would have trouble scratching you.
Personally, I find the Short Sword to be the most versatile weapon in terms of cutting strength. The human sword seems duller when it comes to beheading ogres and the like.
Killing a megabeast (or a titan) typically elevates you to legendary hero status, where every single civ, minus the ones you've made enemies with, suddenly knows of your deed and becomes slightly friendly with you. By default, if you're a legendary hero, goblins will still ambush you, but you can go talk to them and even recruit them. As for the tons of civ thing, it's a worldgen thing. You can set the amounts of civs in a world using custom worldgen, or you can use the worldgen creation parameters outside custom worldgen to get a fewer number of civs (but using that isn't as specific as custom worldgen.).
By default (without modding), clothes provide little, if not nothing at all, defense against enemies. It will, however, provide defense against contact syndromes should you happen to step into a puddle of it.
Currently, yes. Every single quest is to kill something, but Toady plans to add more variety to that.
In order to butcher, you must be standing over the corpse, and wielding a sharp weapon. Press 'x' to open the crafting menu, and there, you'll find the butcher command.
Races don't matter in memorials, so there's a chance that your guy is going to be carved into dwarven history. My mind is fuzzy on this matter, however, and I might be wrong with this.
Pardon my bad english.