I have taken a fast run through the new Codex and here is what I have found:
Standard high level production of the book. Great art and lots of pretty pictures of models. Seems like there is a little less fluff, but I have not read through it yet, so I'm not sure.
I like all the Warlord traits. Living metal got better as did the Doomsday Cannon by a little bit.
Entropic Strike is now just like Gauss Weapons but for hand to hand attacks. yes it does lose some flavor but much easier to keep track of during the game.
A lot of units got cheaper. the Praetorians and Lychguard by a substantial amount. Only the Wraiths, Scarbs, and Night Scythes went up in cost.
The Stalkers squad size now 1-3 models. Yay! Tomb Blades options are cheaper and the squad size in now 3-10.
Destroyers can only upgrade 1 model to a Heavy Destroyer, but you can now take a squad of 1-3 Heavy Destroyers as a Heavy.
Crypteks work a bit differently now. Alone they are a HQ choice. There are a few ways to take several in Formations and the new and very different Necron Detachment system. The Crypteks are no longer restricted in the wargear they can take, they have full access to the Technoarcana and Artefacts of the Aeons lists. And any unit with a Cryptek attached gets a +1 to Reanimation Protocols, so very useful guys.
The C'tan have four entries. Nightbringer and the Deceiver are unique elite choices. The Transcendent C'tan replaces the older C'tan Shard and is now a heavy. The Tesseract Vault joins the Codex in a Lord of War slot. There are six C'tans powers are now, and they are all in the form of shooting attacks. The Powers are rolled randomly before each shot and they are all pretty powerful, some more than others. Each power has two stat-lines, one for Transcendent and one for Coalescent. The Coalescent profile is use only for the Tesseract Vault and has double range and more shots or bigger blast, depending on the weapon.
I have saved the worst for last. All the rumor guys were right. This Codex is lame because they have completely nerfed... the Tachyon Arrow! Sure it is still Strength 10, AP1, but the range have been reduced by, um, I'm not sure how to subtract anything from infinity, but it must be a huge number.
Now the range is now only 120"! How am supposed to I shoot people across the solar system?
They Are Legion
And make Awesome Metal Album covers