Well it looks like a lot of bloggers have taken a leap of faith into the whole death knight subject. With some preliminary information about skills / talents released there is a lot of speculation going on... on... well pretty much everything.
Will the DK improve the tanking situation?
will everyone roll a death knight on the first day or level a pre-existing char to 80?
Will the death knight make specific classes obsolete?
I am not going to indulge in any speculation. Blizzard will take care that the new class 'fits' into the scheme or over a few dozen patches will fix what they perceive is wrong.
A word of advice to anyone who cares little about speculation and more about the reality of things:
Before the release of the expansion get your characters off of servers that run Northrend and the DK starting zone if you actually want to be able to play at all.
Whether or not people will play the DK extensively and how this affects class balance remains to be seen. One thing however is sure: The day the Death Knight becomes available a lot of people will roll the Death Knight or start their invasion of Northrend.
And wherever there are a lot of people grouped together there will be... you guessed it: oodles of latency.
People will be climbing all over each other to get their quests done, people will be standing around like idiots waiting for mobs to respawn and it'll be one big huge clump of death knights at any potential point of intrest.
The technical aspect of things suggest that where there's too many players there's also bound to be performance issues for the server and potentially crashes.
So unless you don't want to play at all come the expansion, make sure at least some of your characters are safely parked in now outdated and soon to be abandoned zones like outland.
From what I read Azeroth doesn't have a dynamic spawn system either (one that adjust to player density) which will make for even more standing around.
So there we have it, my evaluation of the Death Knight... and I think I'll leave it at that until there's actually something more to report than ZOMG RUNES.