I'm going to create a running list of all of the interesting leaked tidbits extracted from the latest Diablo 3 MPQ. More details here and here if you're not already aware of the latest leak.

A lot of the extracted data is development related too, so you can see a brief view of potential future projects. Most of it will show you monster and boss names, zone names, and all of the player skills and abilities. I'm more interested in neat little things (like what you'll see below).

PVP Game Types

* Cooperative
* "Diablo 3" / Normal
* DotA
* Arena
* Team Deathmatch

It's interesting that they're bringing a DotA style game into Diablo 3. You should see this hitting gaming news sites later today.

Random Features

* Looks like there's a targeting mechanism (toggle key) for selecting the more injured monsters first.
* You can drop your banner for people to see (like an emote) and show off.
* Speaking of /emotes, here's what is available so far: Help, Follow, Give, Thanks, Sorry, Bye, Die, Run, Wait, Go.
* There's a gambling Gheed-like vendor available. This was one thing a lot of people were asking about that Blizzard never confirmed (probably because of the "gambling" and real money implication). Any bets on whether this will be available in the final version?
* You carry a Bestiary and a Lore Book built from all of the random book drops that you can collect throughout the game.
* You can receive notifications when someone is inspecting you. Very useful for World of Warcraft, so don't be surprised if you see that feature added.
* Easy quest selection, just click on the NPC and you can walk away. If you chose to listen, you can stick around though. No more walls of text, and you can jump right into the action. You can also listen (yes, listen not read) to quests or lore items while you're on the go.. a nice little feature borrowed from other FPS/RPGs.
* It appears that leveling up causes a nice little "explosion" effect around you, and for added effect it also causes AOE damage to mobs around you.
* NPCs talk WAY too much when they accompany you (hirelings, quest NPCs, escorts, etc). There is probably an option to turn it off.. or at least, I hope there is.
* When you exit your Campaign Game, you can save your progress and return to it later. In D2, if you started a new game, everything was reset. Would be very cool if you could invite people to your saved game (say you came across a very rare random event.)

Items

* Item Reforging: "Transmutes all properties of a single magic, rare, set or legendary item into an entirely new set of properties. This process permanently reduces the maximum durability of the item by 10% and cannot be used on items with required level 50 or above."
* Your gear can be dyed. Very nice.
* Gear can also be "Undyed" and "Vanished" which apparently makes gear invisible.
* Players can purchase Stash Expansions for extra storage. You can purchase "Slots" and "Tabs".
* Crafting Plans are random drops for making weapons or armor, but there are also a ton that are basically Enchanting Recipes. Many of them look like they're named right out of World of Warcraft, for example "Enhance Gloves: Superior Brawn".
* There are a lot of Crafting Plans for Gemcrafting ("Design: Radiant Round Emerald") as well.
* When loot drops, you can set timers on how long you want the item text to appear. That way, you're not always alt clicking to see loot.
* Quivers can also come with special abilities for your bow (e.g. fire damage arrows, snaring effects, etc.) Will be the same for secondary weapons, like shields too.

Special Events

* There's a spiral event (which most likely takes place in the Hell Spire in Act 4) where you can keeping moving from one level to another, in either a timed event or a difficulty progression. The more you progress, the better reward you receive. Reminds me of some jRPGs.
* Each zone can have random dungeons or timed events that popup. There are randomly appearing caves, ruins, temples, towers, scavenger dens, mines, cellars within structures, puzzles, and NPC vendors. There's also the Pony Carnival event and the "Secret Bovine Wundercave".
* There's a timed "Gateway to Hell" event. Also, one where Diablo is trying to corrupt the Crystal Arch (Act 4). Speaking of which, I see a lot of references to Black Soulstones (aka Worldstone Shards) that have been cut or added to the game frequently.. maybe they have more expansion pack plans for them.

Character Naming

* 1-32 max. characters in size.
* Spaces and no special characters permitted.
* The banned character names list is a good read.

Future Stuff

* Act I, II, III, and IV are mentioned. As well as Act V and Act VI. (Which we already knew they had planned to release within 3 years.)
* Player titles go up to level 100, in 10 level increments. I suppose they could be PVP titles.. but why the specific character levels? There are also "Corruption" titles for those with evil karma.
* The beta client has a Verisign expiration date of 12/05/11, identifying a retail release prior to Christmas 2011. Of course, all of this was already released by Daeity a long time ago. :)

Hirelings and Pets

* Hirelings can learn skills as they level.
* You can summon a friendly creature and roams around and collects gold for you as you travel.

Shrines

* Pretty much any kind of shrine you can imagine is there; the typical elemental resistance, health, and regeneration.
* Also a Luck Shrine, a socketing shrine, even unsocketing, item augmentation, Identify shrines, Remove Curses, +25% Magic and Gold Find, +25% experience, etc.

Blizzcon

* A lot of references to Blizzcon since this was (and will be) the build that players used during Blizzcon.
* There's a hidden "BlizzconPrize" that players can randomly loot with the message: "Congratulations! You just found something so awesome it grants you fame outside the game! Show this item to any Diablo 3 attendant to claim your prize."