Above: A typical day in the life of a Death Knight
In this thread, Blizzard announced today that Blood would be the only Death Knight tanking tree in Cataclysm and that Frost and Unholy would solely be DPS trees. This news was met with mixed anger and excitement. The changes were made in the spirit of making the class easier to balance, since a single class having access to 3 DPS specs and 3 tanks specs provides a level of inter-spec competition over which is best and creates an extreme spec-of-the-month situation. This simplified design will also allow the developers to focus more on other classes and not have to nerf/buff various Death Knight specs so often.
The cons are fairly obvious. Death Knights will have fewer options for how they can reasonably play, and some people will be losing their favored role/tree combination. Many people had specific concerns/frustrations with this new design paradigm for Death Knights.
RE: Blood DPS
Many people mourned the loss of Blood DPS, since they didn't like the Dual Wield and spell focus of Frost or the pet and disease focus of Unholy. I can sympathize with this sentiment, and would thoroughly enjoy seeing a non-dual wield, non-pet hybrid spec arise from Frost/Unholy. If the pet and Dual wield talents are hidden deep within their respective trees, such a thing might be possible if the mid-level talents offer a great level of synergy. Why mid-level synergy? This would be to prevent deep Unholy and deep Frost specs from having access to this synergy.
RE: Dual Wield Tanking
Others mourned the loss of Dual Wield tanking. Perhaps there isn't a loss, though. It would seem to be to be relatively simple for two talents to be added deep in the Blood tree that would allow for both play styles. This would be similar the Combat Rogue and Arms warrior models where there are various talents that focus on those characters using a particular type of weapon (swords, maces, etc). One talent would bolster the Death Knight's ability with 2-handed weapons and the other talent would bolster the Death Knight's ability with Dual Wielding. Furthermore, having two talents specifically devoted to this purpose would give the developers easy places to go to for the purpose of balancing the two play styles.
RE: Why Blood for tanking?
Many people were confused about why Blood was chosen to be the tanking class. They pointed to Frost Presence as a reason that Frost should have been the tanking tree. This is a shallow argument, however. To me, Blood seems to be the best choice for the fact that it's 2-hand weapon wielding and self-healing style of gameplay provides a great amount of differentiation from the other tanking classes. Furthermore, is Unholy the most logical choice for a PVP tree? Is Blood the most viable for PVE DPS? Not necessarily, so the argument falls flat.
RE: Why us and not Ferals?
The reason that Ferals aren't also being changed is that there aren't competing tanking specs and competing melee DPS specs within the Druid class. Druids don't have the extra design headache of being in a spec-of-the-month situation, as mentioned above.
RE: So we're losing our self-healing for DPS? How will I level?
Not necessarily, since the tanking talents will be removed from the Frost and Unholy trees, that opens up room in those trees for self-healing talents or buffs to Death Strike (for the purpose of self-healing). Furthermore, Frost and Unholy can each have their own unique method of self-healing. For example, Unholy could leech life from it's empowered DOTs. I don't have an idea for how Frost could self-heal in a way that's consistent with Frost design, but that's probably because I'm not a Blizzard class designer.
How will we benefit?
Currently, all of the Death Knight specs are very similar. In order to simplify the balance process, the developers had to homogenize the Death Knight specs so there were fewer variables for which they had to account. Having this dedicated-tree approach to Death Knight design will allow the developers to have each spec have more varied play styles, instead of them being slight variations on each other. Currently, Unholy's perma-pet mechanic doesn't really do much, but now several things could be added to Unholy that will make the pet more interesting and dynamic (and therefore, hopefully more fun). It will allow each spec to be more focused and different from the other specs.
For example, Blood Death Knights use Death Strike, Frost Death Knights use Obliterate, and Unholy Death Knights use Scourge Strike in order to use spare Frost/Unholy runes. However, these strikes aren't REALLY that different, they are just used to do damage. Sure, some of Scourge Strike's damage is Shadow damage, and Death Strike also heals the Death Knight, but these aren't serious differences when embroiled in melee DPS. For tanking the healing of Death Strike is significant.
The loss of our favored spec/role combinations is great, but I personally feel that Death Knights and, by extension, the game will be better off for it. I gladly welcome the design shift.