So we know the talent lines are going to be Blood, Frost and Unholy, which are also linked to the runes the DK will use. It seems obvious, that special ability within each talent tree will require Runes of their specific type.
In a way, this is a specialistion on top of a specialisation. Not only do you put your talent points in a specific tree, but you also need to pick the proper runes. It's going to be interesting to see how 'flexible' a deep spec Death Knight can or can't be when it comes to using 'off spec' abilities.
After all, if I 'need' to have all unholy runes selected in order to effectively use the last few active talents of the unholy tree, then this prevents me from using any core Blood or Frost abilities.
Unless of course, you can change your selected runes on the fly, which might allow for somekind of 'twisting'.
As for the talents, I think Death Knights are in luck... because they have more than 1 purpose. Class that have only 1 purpose (i.e. Rogues, Hunters, Mage, Warlocks = purpose is damage) usually end up with 1 strong pve talent tree, 1 strong pvp talent tree... and one talent tree that's kinda 'meh'. The 'meh' tree may change with gear level or even every other patch, but there's always one.
Look at a class like warriors - They can tank and do dps. Much like the Death Knight. The Warrior's 3 trees are pretty clean cut in their purpose, Tanking (Prot), PVP Dps (Arms) and PVE Dps (Fury).
While this does limit 'hybrids build', it also makes the class less of hassle to spec and streamline. I guess there's a possibility that Blizz will somehow make all 3 specs viable for tanking, dpsing and pvp... but I wouldn't hold my breath.
So what little we know of the deathknight's abilities can be summed up as:
Army of the Dead - Spell summons undead minions (ghouls) to fight for Death Knight. These creatures run around and attack things at random.
Unholy Embrace - Target is embraced by a silhouette of darkness for 6 seconds, corrupting any healing spells and effects cast upon or currently affecting the target to cause damage for 50% of their healing potential instead of restoring health. Any damage shields cast upon or currently affecting the target will also be instantly consumed, dealing 50% of their absorption potential in damage. Once afflicted with Unholy Embrace, the target cannot be afflicted with Unholy Embrace again for 1 minute.
Death and Decay - An area-of-effect damage spell.
Blood Boil - A damage-over-time ability that after the duration refreshes two of your blood runes.
Death Coil - Bornakk commented on this:
We understand that Deathcoil is a spell Deathknights had in Warcraft 3. While things may change, we simply haven’t made a final decision regarding this yet.
My guess is...
Unholy: Army of the Dead, Unholy Embrace (I suppose it could go with Blood, but the name makes it kinda obvious), Death & Decay, Death Coil
Blood: Blood boil
Frost: ???
Going of this, my guests are as follow:
Unholy will be the pvp tree. Unholy Embrace is a pvp ability and it'll probably be the key ability for DK in pvp. Slap this on a target with low Health and burst it down and it's GG. The ability to summon undead might sound powerful, but it really depends how they handle it... it could be on a long cooldown or they could be very weak (think Resto Druid trees).
Blood will be the dps tree. Not that much to go on that one, but Blood Boil sounds like the quintessencial dps move - a Damage over time that 'refund' the energy used for it. I expect the DK to have many 'bleed' effect and possibly way to increase damage, this will make for a very easy synergy with rogues (Rupture) and Feral Druid (mangle, lacerate, shred).
Frost will be the tanking tree. Going both with a process of elimination and logic - the mage's frost tree is all about survability and making oneself harder to kill. Typical frost abilities usually increase durability or slow down the ennemy. Deep frost DK ability may be able to slow down the attack speeds of bosses, which could be the key to their tanking mechanism. Frost will also probably control the DK's snare... smart design would be to put some of the snare/armor low in the frost tree, to make it a good 'off tree' for pvp, while keeping the tanking mechanism abilities deeper in.