This thread is for collecting spoilers about new, closed prerelease stuff, so we don't have to go looking for it in a bunch of different threads. Without further ado:
Gargantuan rat, mist wolf, clay statue.
Fairy dragons now breathe all elements in addition to water.
New boss monsters
Check this thread for the community's findings. You can also check this wiki page.
New artifact rooms
See this thread for the community's findings on this.
long spear / mithril long spear / adamantium long spear / eternium long spear
full plate mail / mithril full plate mail / adamantium full plate mail / eternium full plate mail
lump of clay
mithril short sword / adamantium short sword / eternium short sword
trollish war club
sabre / mithril sabre / adamantium sabre / eternium sabre
rapier / mithril rapier / adamantium rapier / eternium rapier
fencing gloves of fidelity
fencing gloves of speed
sabre of swiftness
sling bullets of giant / humanoid / undead / construct / dragon / insect / plant slaying
New chaos knight crowning gifts
Spiked armor of chaos and terror, Foeslammer, Moloch's Thorns, spiked crowned helmet of chaos lordship
Mist elf racial crowning gift
Staff of creation
New duelist crowning gifts
Fencing gloves of St. Montojja, rapier of the needle
There has been no report so far of finding these artifacts randomly in-game, which, if true, is a bug (reported).
Thunderstroke and True Aim now return when thrown.
The silver key can be 'u'sed to create a door for about half your current pp.
The boots of great speed now give +100 speed.
Possible crowning gifts for new classes
Chaos knight: Spiked armor of chaos and terror, Foeslammer, Moloch's Thorns, spiked crowned helmet of chaos lordship, Grod, Death's Blade
Duelist: Fencing gloves of St. Montojja, rapier of the needle, Kinslayer, Executor, Bugbiter, Ironfist
Also priests are no longer crowned with Justifier (since p1). No data yet on which artifact replaced Justifier in the priest crowning gift pool, but it was not a new artifact.
Chaos knights start very chaotic and it is very difficult to steer their alignment away from chaos. While chaotic, most NPCs will refuse to interact with you. The ones that will talk to you are listed as follows: ghost librarian, Lawenilothel NPCs, demented ratling, old crone. All others will not talk to you and Hawkslayer will turn hostile if talked to (you can still give criminals to the sheriff for gold). These effects can be circunvented by converting to a different alignment. If not chaotic, all class powers will be turned off, you get -2 to all stats, and damage will be dealt to the PC periodically. This damage can be circumvented by fire resistance or immunity.
Chaos knights start the game with 4 corruptions.
Heir gift: chaotic mithril battle axe of slaughtering.
A chaos knight does not need to be extremely corrupted to enter the chaos plane.
To use special class powers duelists must be unencumbered and wear armor not heavier than 250 stones. This includes the powers shown with ctrl+x, the chance to penetrate armor and the additional bonuses to to-hit and damage with your wielded weapon (which depend on weapon skill level).
The class gets good bonuses to starting Dexterity.
Heir gift: masterwork <starting weapon> of defense.
Heavy biases in weapon skill level advancement. Shields, unarmed fighting, polearms and 2H weapons require 140% more (36 marks for L2); all other 1H weapons require 20% fewer marks (12 marks for L2). Missile weapons are ridiculously expensive to train - 75 marks for L2.
Duelists appear to be completely ambidextrous (you receive no penalties for fighting left-handed).
Slapping a non-hostile monster with gauntlets (level 18 class power) is not a chaotic act (and is a good way to make them hostile!). Throwing gauntlets at a monster has the same effect as giving them.
Starting alignment is N. On first observation ratlings seem to not require a tremendous amount of food so it is probably safe to say that they need either the same amount as humans or less.
Starting bard pet: gargantuan rat. Preferred sacrifice: artifacts.
Starting alignment is N. Mist elves require considerably less food than other races. They require more experience to level, but not as much as trolls.
Starting bard pet: mist wolf. Preferred sacrifice: spellbooks.
Mist elves seem to take half damage from melee atacks. Don't know the numbers exactly and whether this is calculated after or before substracting PV/Immune to Pain talent etc.
They have a very small amount of hitpoints compared to every other race, which makes them especially vulnerable to traps in the early game. This makes it a very good idea to pump Alertness, get Detect Traps, wear rings of searching, get fire and acid resistance and being generally cautious.
Mist elves are damaged by wearing/wielding iron items. This includes artifacts. I believe one item will deal one damage per turn; don't know if the formula is different in the wilderness; don't know if multiple items will stack for more damage. Wearing one ring made of iron will not cause damage, but two will. Wielded items will not deal damage if wearing gauntlets (but make sure the gauntlets themselves are not iron).
Scrolls of magic mapping no longer work on the bottom level of the minotaur maze, but the spell does.
Rewards are now greater. Grobblewobble reported an axe with higher damage, one random artifact, 4 PoCC, 6 PoGA, 1 potion of strength, 3 spellbooks.
The snake from beyond now sees in darkness and apparently is also now immune to blindness.
The ancient chaos wyrm is now resistant to confusion and paralyzation.
Yulgash and the ancient stone beast have been beefed up. The ASB has slightly higher PV and now also throws missiles. According to the creator Yulgash and Nuurag-Vaarn now can teleport while on the temple levels. We're still waiting on reports of that actually happening. Perhaps they will if blinded - or perhaps only if injured.
New game mechanics
Traps are now completely random everywhere (no "top trap type" calculating possible).
Controlling teleportation will often fail in Darkforge. There also might be guards in the treasure rooms.
Confusion can no longer be safely waited out by cleaning your ears. However, changing equipment (like loading different missiles) still works. You can also apply the Listening skill.
First aid doesn't work while stunned anymore.
On average Thrundarr will give more mid-rarity monsters for his first quest.
Summoned monsters and duplicated gremlins no longer drop loot (but summoned monsters still drop corpses). Gremlins no longer have any suitable pockets.
Dipping a stack of rings in a potion of exchange will exchange each ring individually.
Monsters are more likely to squeeze past each other. Summoners especially seem likely to hide behind their summons.
Herbs, crystals of knowledge, carrot juice and many monster corpses no longer raise stats past their potential maximum. Burden training still allows raising St up to 18 although, according to the creator, training stat potentials is now a slower process.
Casting from HP now abuses Toughness and Willpower in addition to Mana. Concurrently, scrolls and crystals of power now scale better.
Gardening can now be used to obtain herb seeds if applied to a blossoming herb bush. If successfully applied herb seeds are added to your inventory. If not the bush is destroyed; you can apply gardening on a bush repeatedly until you destroy it.
Darkness and invisibility now behave differently: monsters will no longer stay put in darkness when attacked, and when moving around will attack you if they happen to try to move into your square no matter if you are in darkness or invisible.
The candle healing bonus has been slightly nerfed and now lets you recover an extra 2 HP every 20 turns.
The gods are now not so willing to accept herbs as sacrifice.
The PP cost of the teleport spell is now affected by danger level.
Monk armor penalties seem to be calculated by armor weight now instead of armor type.
The dwarven mystic now also gives a ring of ice to neutrals.
Gambling in the casino now corrupts. Robbing the casino is now far more dangerous and gives a huge alignment drop. The big casino guard will turn hostile if you try to teleport him.
The uberjackal effect is gone.
Something has changed about taming animals, but it needs more testing. Grobblewobble says: It seems that when an animal is "more friendly now" and you apply music again, there's a chance the animal will turn hostile.
Successfully applied First Aid now scales much better with higher HP totals.
New corruption considerations
The following is only applicable to post-p5 versions
You will get a bonus of +1 to St, Dx, To and Ma and -1 to Wi for every four corruptions. If you are a chaos knight this applies every two corruptions, unless your alignment is not chaotic - then it applies like other classes.
The following is also applicable to p4 and p5
"You are attached to chaos corrupting your foes" will fully heal WMoPC whenever a hit connects. I don't know if this applies after the damage dealt; if it does, the only way to kill one in melee with this corruption is to one-hit it; if it doesn't, it makes them impossible to kill. You can also turn enemies into chaos beings with the corruption dealt by your attacks if you deal enough corruption before you kill them; this includes turning into WMoPC. Be careful with this as it will greatly complicate your life if you turn an orb guardian into a WMoPC. Then they will have to be disposed of through other means. In p5 this would make the game unwinnable as they wouldn't drop the orb when killed, but this was fixed in p9.
"Your mouth has turned into a writhing mass of prehensile tentacles" will make speaking to NPCs fail most of the time, but not every time. Keep trying if you really need to talk to someone. It will also make spellcasting fail a lot.
"Your bodily tissues seem to be in a state of decay requiring a lot of will to stay up" will greatly reduce your natural regeneration rate, to the degree that it will at times seem to have stopped completely.
Other outstanding questions
(None at the moment)
Outstanding bugs (not an extensive list)
(None at the moment)
(Data from personal research and lifted from various threads; credits for text nearly completely lifted: BenMathiesen, Dogbreath, grobblewobble, JellySlayer, Arbot, Kyreles.)