1) I don't know off the top of my head; the most important ones are the Agnostic Collar, Trisword, and Gloves of Midas, which respectively are unlocked by the Monk-exclusive, Warlord-exclusive, and Rogue-exclusive levels. Soul Orb is unlocked by the gnome village quest, which is another important item.
2) Goblins are found in the south, Orcs to the west. Just random chance. I found goblins on my first venture to the south, but I haven't found the Orcs yet.
3) To unlock Naga City to the south, you must first kill Stheno in the Hexx Ruins. Then play until you find a sub-dungeon filled with Naga, which will unlock the Ick Swamp level. Once you beat Ick Swamp, this unlocks Naga City. To unlock Dragon Isles in the west, just beat the Labyrinth with any class. To unlock Namtar's Lair to the east, beat the Halls of Steel with any class. There is also vicious-mode Halls of Steel, which is unlocked after beating hard-mode with four different classes. To the north, you unlock Demonic Library after completing the Shifting Passages with any class. Finally, vicious mode Gaan-Telet is unlocked by defeating hard-mode Gaan-Telet.
The Dragon Isles is the only vicious level with an important reward (the Dragon Shield).
4) Not sure about the Tinker's prerequisites, but you'll find him in a sub-dungeon that is hidden behind walls. You will need ENDISWAL to access it. I'm not sure what the prereqs are for him to appear.
Not sure if you're interested, but here's how deities work:
Glowing Guardian: found in the East
Dracul: found in the East (prerequisite: glowing guardian)
Mystera: found in the North
Jehora Jeheyu: found in the North (prerequisite: mystera)
Taurog: found in the West
Binlor: found in the West (prerequisite: taurog)
Tikki Tooki: found in the South
Earthmother: found in the South (prerequisite: tikki tooki)
Pactmaker: found anywhere (prerequisite: Unknown [anyone know?])