How far we've come,
In WotLK Death Knights were brand new. As such, at the beginning they had severe balance issues. As WotLK progressed, however, it was easy to see that Death Knights were suffering a bit of an identity crisis. Every single spec could successfully fill a DPS or Tanking role.
The scary part was how incredibly good they were at both.
As Cataclysm loomed with the newly revamped talent trees, the Blizzard developers saw an opportunity to separate Unholy, Frost, and Blood once and for all, giving each spec it's own distinct flavor.
Unholy became hallmarked by its usage of diseases, pets, two handed weapons and shadow damage.
Frost became a spec known for its massively damaging hits, AoE, frost damage, and careful management of both procs and resources.
Blood became a tanking spec exclusively, and was given the unique mechanic among tanks to rely on active self healing and shielding.
When Cataclysm dropped, however, it became quickly (and painfully) obvious that Frost's gameplay was boring and subject to a ton of random chance. Blood's gameplay was difficult and often ineffective. While Unholy's gameplay was in many ways more enjoyable than the other two specs, it was uncompetitive for DPS when compared to other classes and specs.
Where we are,
As Cataclysm progressed, Blizzard worked to resolve these issues (after a good bit of pushing and complaining from the Death Knight populace). As it currently stands Blood is a competitive tank, reasonably reliable and remarkably fun. Frost's gameplay is still quite random, but overall it has improved quite a bit and has excellent, acceptably reliable DPS. Unholy (like the other specs) still has its problems, but its damage is again competitive, and the T13 set bonus alone has gone a long way towards making the gameplay more active.
(The Bloody Tanking, Unholy DPS, and Frosty DPS posts from this blog go further into this topic)
But with Mists of Pandaria just around the corner, we Death Knights have an uncertain future.
Where we're goingThe beta has dropped, and with it there have been some huge changes/additions to Death Knights. Here are a few of the more pressing concerns. (I will not be discussing several problems with DPS all the Death Knight specs are suffering on the beta, as those will almost certainly be addressed before Mists is released.)
First- With both Soul Reaper and Death Syphon (two new Death Knight spells), Blizzard is making Death Runes an odd sort of fifth resource (Runic Power, Blood Runes, Frost Runes, Unholy Runes), and one which we have to carefully earn and spend. This is probably going to lead to a variety of "Rune Tetris" as it is called in the community- where Death Knight's primary fight mechanic will be creating and maintaining specific sets of runes and reacting to which ones are currently available with specific abilities.
Second- The introductions/revamps of Conversion, Runic Corruption, Runic Empowerment, and Blood Tap all grant a different flavor to the default rune resource system. I must say that as they stand, only one of these seems to actually improve the default quality of gameplay- the others will instead introduce further complexity as a trade-off for either getting the runes you want and/or control over when you get them. Might I just add that as Unholy we currently have to watch three separate procs/stacks diligently, a couple of buffs, four separate resources, and manage our diseases? I personally would happily sacrifice some DPS to not suffer further over-complexity.
Third- With the introduction of Glyph of Shifting Presences along side the huge changes to Frost Presence and Unholy Presence, we will likely have to "stance dance" or be starved of resources (Frost starved of Runes, Unholy starved of Runic Power)
While I'm certain that not all of these changes will make it into the final release of Mists as they currently exist, we can use this information to hazard a few guesses as to Blizzard's intent for overall design changes to Death Knights in Mists of Pandaria. The majority of these changes seem to indicate that Blizzard wants resource management to become far more critical than it is today. From the abilities made available to us so far, it seems that Death Knights will have a few 'core' abilities they use whenever they can (either for DPS or to survive), and in the resource starved time between these abilities, Blizzard wants Death Knights to earn resources back thorough various means to continue using those core abilities.
For now, I'm feeling a bit pessimistic. I truly hope that whatever Blizzard does decide to do, Death Knights will continue to be as fun and as interesting as I've always found them. Hopefully we'll understand more as the Beta continues.
Huge thank you to Tsukiyuri, Heartless, Magdalena, and countless others in the community for all their hard work cataloging, discussing, and evaluating the Mists of Pandaria's changes to Death Knights. This post stands upon their hard work.