Info here about Doom 4 gameplay at QuakeCon.

Well, as many Doom/id fans know, Doom 4 was only shown to QuakeCon attendees. They checked for cameras and google glass during the presentation, but we have yet to see if maybe someone snuck by with those sunglasses-cameras or mini cameras or something. Luckily, there are many websites/forums that have people who replied to/made articles/threads that go in great detail about the game.

Looks like the main points about the game is:

It uses a new version of id Tech, called id Tech 6 (some call it id Tech 666)

It is going back to it's run and gun roots

Has absolutely brutal fatality moves like Brutal Doom, and the game is very gory

Plasma gun, Double barrel, rocket launcher, pistol, shotgun and chainsaw were shown

Two levels were shown

Game is a reboot, not a sequel

Monsters look more "disgusting" and "threatening", but somewhat resembling of Doom 3 a little

Megatextures are GONE!

1080p 60 FPS on next-gen

A Parkour element is in the game

Movement similar to Quake 3 Arena

Graphics "aren't groundbreaking, but are a lot better than RAGE or W:TNO"

NO REGENERATING HEALTH AND NO RELOADING

Check the links below for more details
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In-depth articles/threads I found:

Kotaku

Doomworld Forums: Doom reveal first impressions

Doomworld Forums: Doom 4 at QuakeCon

PC Gamer

VG24/7

Eurogamer.net

GameSpot

IGN

I will try to link leaked gameplay if any is found.