Death knight DPS
Tanking is not the only aspect of PvE that death knights have held a significant impact on shaping: there is also the wide changes that death knights have brought on the DPS scene. WotLK has been an amazing change of philosophy for WoW with the bring the player not the class (BtPntC) design, yet it has also caused many of the issues that we have seen today. BtPntC hinges upon two distinct principles of equality: damage equality and utility equality.
To many players, damage equality is of the utmost importance. I think we can all agree that damage is one of the key driving forces of virtually every combat based game out there; healing is nice, soaking damage is all well and good, but it is the raw power of a character, it is their ability to blast things to bits, that is really exciting. Death knights have shown us many a problems that can exist within damage equality, particularly in regards with melee attacks.
Unholy has been one of the prime DPS specs of choice throughout much of the death knight community. Blood and frost are to be respected, and they've demonstrated many things in their own right (in fact, frost is my personal favorite); however, it's been unholy that has demonstrated a key facet of damage the most. I am, of course, referring to mixed damage attacks -- those attacks which are based both physical and magical. Scourge Strike more than anything else has been the best indication of how difficult it is to properly balance out these effects. Magical damage is based around the principle of how it ignores armor; which is an excessively powerful mechanic that often goes ignored by many players. Allowing a melee class to wield magical abilities isn't a strange or new concept, but it is a difficult concept to balance effectively.
At its most basic premise, original RPGs operated in a sort of rock-paper-scissors method; magi beat warriors, warriors beat rogues, rogues beat magi. In the beginning of WoW, that was actually the basic premise of how PvP operated - more on this later - however WoW has evolved beyond such a basic form since then; melee classes overall are rarely restricted to physical attacks. Aside from warriors, nearly every melee class in the game deals some form of magical damage. Death knights moved further beyond what any other melee class has before; they are the perfect combination of physical and magical. Only blood differed from this in any way, which is why it isn't surprising that blood is becoming a tanking tree in the next expansion. By focusing solely on the duality of the death knight class, Blizzard has shown they can make this duality work, but it takes time, it takes work. This is often frustrating for players, understandably so, but in this case we must have patience.
Beyond damage equality, there is also balancing the utility that classes bring. This is the second principle of BtPntC; spreading out utility, homogenizing utility, in an effort to allow for leeway in class choice when it comes to filling out a raid group. That being said, we have quickly learned that not all utility is created equal. It isn't merely enough that various classes are able to provide a specific buff within whatever capacity it is that they can; the method in which that utility is brought must also be done equally in order for the philosophy to be successful. Although death knights might not be the perfect example of this, elemental shaman with Totem of Wrath and demonology warlocks with Demonic Pact might be better, the lack of power in the death knight's unbalanced utility is predominately caused by the lack of encounter design to support it. Ebon Plague is, at the core, perfectly balanced in respects to Earth and Moon, although both are far superior to Curse of the Elements, but Ebon Plague has one mechanic which neither of the others do -- the capacity to be spread to multiple targets with only a single press of a button.
Pestilence and the way it interacts with Ebon Plague is a huge breach in the equality of class utility. If there had been any examples of boss encounters with a significant number of clustered adds where this imbalance had made a difference. I do not feel that I can stress the capacity for this mechanic to have been such a stringent requirement enough; had a more creative workaround not been developed, unholy death knights would have certainly been required for dealing with Vile Spirits on heroic Lich King. Even though it never really became a major issue, Blizzard learned quickly of this issue, sadly, though, they haven't addressed the core of it. They are, slowly, with Cataclysm, and that is a good change.
The PvP death knight
As much as I postulate on the PvE aspect of the game, that is but a fraction of everything that encompasses WoW. Although I am not a PvP expert in the sense that I don't carry around a gladiator rank, nor do I have a top rated team, I do follow the balancing mechanics of PvP very closely. The most interesting thing about PvP balance is how everything hinges upon such a thin thread; even minor little changes to a certain class can have a widespread impact on what teams, classes, and specs are considered to be viable. For everything that the death knight class has taught us about PvE balance, we have learned so much more about PvP balance from them.
Death knights were created virtually as a direct counter class. During the last days of TBC PvP, restoration druids were the king of kings when it came to healing. Their strength now is debatable, but back then no one could deny the awesome power that druids held. Blizzard made a very big slip when they originally developed death knights -- they were clearly made to overthrow the druid overlords of the time.
The flaw of counter classes
Designing a class to directly counter another class is a terrible design plan. This may not seem obvious, or perhaps it does to you, but within a PvP system as complex as WoW's counter classes simply cannot exist. To some extent, it may seem sensible; however, it could quickly turn into an arms race of how many classes a specific spec can counter, and then there's the class representation imbalance as well. There's a very good reason why PvP cannot be balanced around the high end. Beyond the basic flaws such as skill or other human reasons, there is the most basic human reason of all; that certain players of certain classes are simply more likely to engage in PvP than others. Just to throw out an example, say rogues are more likely to play in PvP; this would mean that any class or spec specifically designed to counter rogues would be at an advantage in terms of creating a stronger team, which would lead to a population imbalance against rogues and their direct counter classes. Eventually this would lead to a higher number of players switching to the class that counters the class which counters rogues, and so on and so on would the cycle repeat itself.
Each class needs to be designed to have certain flaws, certain weakness, and also certain strengths that they can capitalize upon. These weaknesses and these strengths can make them better suited at handling certain classes but never should it allow them to so dominate another class that they will always win no matter the variable of skill. Death knights have taught us this excessively. They were designed to counter restoration druids, and it showed. The representation of restoration druids didn't just drop significantly, it cratered, they completely fell off the face of the earth, or Azeroth if you like. This is why death knights were changed, for better or worse: because Blizzard realized that designing classes to specifically counter others is not a wise choice.
The dispel problem
Dispels are a huge PvP issue; they have been for a very long time. Ever since the on set of arenas, dispels have been under attack for the power that they hold. Blizzard realized this at the time; they thought about change, yet nothing was really implemented for WotLK. Perhaps, at the time, they figured that merely increasing the number of armaments within the race would actually lead to an equalizing effect; sadly it did not. Death knights were actually rather integral in showing this as they were perhaps the weakest class when it came to being vulnerable to dispels.
Thankfully this is finally being fixed, to what degree we cannot yet be certain, yet we can be certain that Blizzard has learned from the flaws that death knights have highlighted.