In a silly thread in the Druid forums asking whether the druids would get a "dark form" to go along with the Death Knight being a dark version of the Paladin (which it...sort of is, I guess), US CM Nethaera dropped a few interesting pieces of DK information.
[when asked if DKs will start at level 55, instead of 60 or 70] That is the current thinking. It could still be subject to change though and is not concrete. We're still toying with ideas of how we want to make it all work. We want people to feel like they are learning not only the how's of becoming a Death Knight from the lore perspective, but also how to play one.
[More character slots?] Undetermined as of yet. We'll let people know when we know more.
[...the current feeling is that a player with any class over 55 (or whatever the level ends up being) will be able to unlock Death Knight? ] yes. That is our current thinking.
[asked about balance] It is going to be balanced against other classes and will not be able to stomp everyone else just because it's a Death Knight. Yes, they will be powerful but in their own unique way. What makes it a Hero class is that it is from Warcraft lore as a Hero class and also you will get access to it at a higher level. Any race can be a Death Knight in our current thinking as well since the path to corruption can be founded with good intentions much like what happened to Arthas.
[How many abilities will DKs start with?] Undetermined as of yet. We're still fleshing a lot of things out. This is another reason we caution people about the perception that the expansion is coming out 'too soon'. We have only shown people the tip of the iceberg and have a lot more that we are still working out and planning for. Once we have more details, we'll be sharing them so people can discuss it more and plan for the future.
So there's a lot of "not sure yet" in that post, but there's also some good information, notably that they currently think DKs will start at level 55. That's a bit lower than I would have expected. 58 or 60 would make more sense, in my opinion, in order to be able to start out in Outland (but then again, does that make sense lore-wise?). So how excited are you about Death Knights? Are they your main reason to buy Wrath? The change that will kill WoW?