Besides lots of fluff and more-of-the-same, there is one real innovation in the announced Wrath of the Lich King second expansion for World of Warcraft: the first hero class, the deathknight. What was announced was that you first have to level one of your regular characters to 80, then do a quest comparable to the warlock epic mount quest, and with that quest you "unlock" the ability to create a new deathknight character. No, you don't lose your existing character, you simply gain the ability to make new deathknights of any race. And they don't start at level 1, but at some not yet defined higher level from 55 to 70. So you get to play through Northrend, and maybe Outlands too, again, but not through the old world. Deathknight will be tank / DPS hybrids, wearing plate armor, but no shields, and wielding either two-handed or two one-handed weapons. They will use a new "rune" mechanism, where you have a special sword-shaped bar into which you can fit 6 runes, or three different types. Abilities you use make those runes fade, and then they come back slowly. So if you put in 6 unholy runes, you can use only unholy abilities, but got lots of juice to do so. If you mix in some ice runes, you get a wider choice of abilities to use, but can use each ability less often.
Deathknights are going to be extremely popular. Hell, this is the first really new class after three-and-a-half years, even if WoW had introduced the Knights of the Fluffy Pink Bunny Order they would have been popular. The obvious danger is that if lets say deathknights start at level 60, a month after WotLK is released you'll see Hellfire Peninsula full of deathknights, with barely any other class around.
/shout Rampart group looking for more, please no more deathknights, we already got 3 of those.
Might be a good time to level a healer up from 60, you'll get lots of invites. Having over half of the population of Outlands consist of deathknights will feel extremely silly, and not play very well.
At some point the first enthusiasm will calm down, and deathknights will reach level 80 and mix with the rest of the population. And then what? Deathknights will probably make superb soloing characters, dealing good damage while having good survivability. But obviously they won't tank as well as a protection spec warrior. And they won't deal as much damage as a rogue. It's the old hybrid problem, if the hybrid is better than the pure classes in their fields, he would simply replace them. If the deathknight is as good a tank as the warrior, but deals more damage and soloes better, all the existing warriors would be screwed. Same if the deathknight dealt as much damage as the rogue, while being better armored, that would just kill the rogue class. The safe assumption is that the deathknight will be a typical hybrid, and not as good a tank as a warrior, and not as good a damage dealer as the rogue. And then, who is going to invite deathknights to a raid? Unless they have some special abilities needed to overcome special challenges, a tank/dps hybrid just won't be needed. Believe me, I tried to get raid spots as a fury warrior, and that wasn't easy. My guild took me back in the days of Molten Core, where raid groups still had some room for slack. But in a 10- or 20-man raid there is simply no room for a half-tank-half-damager.
My final thought on deathknights is about the available races. Rumor has it that you'll be able to make a deathknight of all existing races. People are already laughing about the idea of gnome deathknight, but actually that idea isn't fitting so badly into the lore. Gnomes do have warriors and warlocks, so an anti-paladin type warrior powered by the forces of darkness wouldn't feel so strange for that race. They might look funny, though. I'm actually more concerned about the lore of lets say nightelf or tauren deathknights. Races that don't have warlocks, because they are attuned to nature, and will have nothing to do with the forces of darkness. And suddenly they get deathknights? That will jar a bit with the lore fans.
I don't know how many hero classes Blizzard is planning. But I think it would have been better to plan for at least half a dozen, and introduce them in pairs, not one by one. WotLK really should contain a healer hero class beside the deathknight. It is not as if the game currently has too many healers and not enough damage dealers. A flood of deathknights is not going to improve that balance. Why not a dark druid as well, being able to heal by sucking life out of nature, and being able to transform into hideous monsters? At least then you could make groups full of deathknights and dark druids and have all the bases covered.