With all the new information swirling across the intertubes about Wrath of the Lich King, I could not, and would not want to, cover it all. That being said, I am very interested in the Death Knight, the first of the upcoming Hero Classes. While a great deal of official information is available about the Death Knight on World of Raids, I'm going to make a few predictions based on what we know so far.
Blizzard, while not setting the number in stone, has said that Death Knights will start at about about level 55 and will be unlocked once a player has a character at that level or higher. Blizzard has also said that all races will be able to become Death Knights, though they have not given a lore reason yet.
My prediction is that The Lich King/Arthas has begun using his vast mental powers to dominate a few citizens of both the Horde and the Alliance, much like the Lich King psychically controlled beings, without them being undead, when he first crashed to Azeroth. Levels 55 to 60 would essentially be Death Knight boot camp, as your character, still under Arthas's control, trains you in the use of 55 levels worth of abilities. Come level 60 there will be a major quest line involving breaking free of Arthas's power, just as the Forsaken did. Perhaps the invasion of Northrend
distracts Arthas and he will lose his focus on his fledgling Death Knights, allowing them to escape. Following their freedom, the independent Death Knights will go to Outland to train somewhere far from Arthas's influence, and will return to defeat there former master come level 70.
As for the Death Knight's talent trees, I'm predicting one Tanking tree and two DPS trees, just like the warrior. I think that Death Knight tanking will involve duel wielding, allowing Blizzard to not build 2h tanking weapons that only Death Knights can use, since Blizzard has show that they are moving towards itemizing classes in more similar ways instead of having loot that only one spec could ever use (see Retribution Paladin reitemization in patch 2.4). Dual wielding will allow Death Knights to make up for the loss of stats from not having a shield, while the Frost Presence, which, according to World of Raids, will increase armor and threat, will make up for the loss of armor. The Death Knight's threat stat will, under this system, most likely be + hit.
As for the two DPS talent trees, I think it's safe to say that one will be geared towards big 2h weapons while the other will be for duel wielding. One of my less substantiated predictions, however, is that, in order to differentiate Death Knights from Warriors, the 2h tree will be for PvE, while the duel wielding tree will be for PvP. Of course, this will make them look like Retribution Paladins in PvE, but we'll see how it pans out.
For the Horde and the Naaru!