I have gotten my hands on some fairly solid Necron rumors. I say they are fairly solid because the source I got them from has proved reliable in the past, however the rumors themselves are indeed 2nd hand (I didn't see the codex myself), and therefore I can't personally verify any of this. That also means if you have any more questions on anything I (probably) won't be able to provide you with any more details (as I don't have them).
Anyway here goes:
1) No surprise here, but Necrons are still essentially Tomb Kings in space fluff wise. A shift in fluff is now that all of the higher up necrons are intelligent while the lower level ones are still basically automatons. I guess that's roughly the same as it used to be, but apparently there is more emphasis in the sentience of the higher Necrons now (hence the named Lords).
2) C'Tan are still in the codex...sort of. They have been moved to the Elites section and are now a single unit choice, but with variable upgrade powers you can take.
3) We'll be Back is no longer called that and it is NOT Feel No Pain. Basically it is a 5+ bonus save similar to Feel No Pain, but doesn't get cancelled out by double strength, AP1/2, etc. In addition, this save is made at the END of the phase after morale tests are taken. If the unit is wiped out or is falling back at the end of the phase, then the save cannot be used. So as an opponent, if you manage to wipe the unit out or make them fall back, then you prevent any Necrons from getting back up...unless the unit has something called the 'ever-living' rule, in which case it gets to attempt this save even if the unit is wiped out.
4) Gauss Weaponry does NOT have rending. It retains the 'auto-glance' on a penetration roll of a '6' rule, but apparently otherwise has lost the 'auto-wound' on a roll of a '6' regardless of Toughness that it used to have. The Gauss Cannon is now apparently Assault 2 & AP3 (I'm assuming the Strength is still 6). The Gaus Flux Arc on the Monolith no longer automatically hits every unit within range, instead each one fires separately and can hit four different targets.
5) Phase out is gone.
6) Destroyers now count as jump infantry and there is a new Fast Attack Jetbike unit called Tomb Blades.
7) Immortals are now a Troops choice. Pariahs are gone. Flayed Ones are Elites.
8) There are indeed 6 named characters (although I have no more info on them). There is also a Necron Overlord and a bodyguard unit called Crypteks. Crypteks can be split off before the game to lead units of Necrons, very similar to Wolf Guard in the Space Wolves codex. Crypteks can take a bunch of different technology upgrades, so there is kind of a way now to have personalized squad leaders in a Necron army.
9) They army is much more mobile now in general. They have several units of jump troops, and 2 new transport vehicles. 1 transport can carry 15 necrons while another can carry 10. Jump pack units count as 2 models and jetbikes count as 5. The one that holds 10 is able to recover casualties to a unit (although can't take it beyond its starting strength). Necron vehicles besides the monolith are open-topped skimmers. However they have something called 'Quantum shielding' which gives their front & side armor +2 until the vehicle takes its first glance/pen, at which point it loses the shielding. One of these two transports (not sure which one) is apparently a personal transport for the Lord (and his retinue).
10) There are six new vehicles in the codex (including the two transports mentioned above). Two of which seem to be based around this old Jes Goodwin design:
http://storage.canalblog.com/29/28/731795/53057637.jpg
11) In general Necrons have a lot more long ranged fire support. There is also a new weapon type called Tesla weapons. These weapons, on a roll of '6' to hit automatically cause two more hits. The biggest version of the Tesla weapons has a chance of hitting all other units close to the target; both friend and foe. One of the new vehicles carries the doomsday cannon: 72" S9 AP1 Heavy 1 large blast. BS4.
12) In the Elites section there is a big new walker. It has a heat ray that that is either a melta or flamer depending on what you want to do. Any unit hit by this vehicle is marked (as in: laser targeted), which means that all other necrons count as being twin-linked for shooting at the marked enemy unit that turn.
