You will notice that all references to PvP, DPS, and tanking are completely stripped away from the descriptions, instead focusing on more specific ideals and ability types. It should be noted that the Presences themselves still remain the same (they've been given a seperate section on the information page now), so maybe it's not as huge a shift as it first appears. However, by specifically removing terms like tanking and DPS from the talent tree descriptions themselves, it seems like Blizzard is deliberately moving away from pigeonholing any single tree into a single role.
If this is what they're doing, and it works, it may be that we will see Death Knights that can spec any way they want, and still have their own unique ways of tanking and doing damage. Much like Feral Druids can tank and DPS well with a single spec, it may be that Blizzard is making Death Knights able to tank and DPS well with any spec. It's a gutsy move to be sure, but if they pull it off, it may help the tank shortage, and it may also pave the way for further talent flexibility.
It may be that Blizzard will stretch this kind of flexibility to other classes as well. It is true that Arms Warriors and Retribution Paladins can already strap on a shield and do some small amount of tanking at the 5-man and 10-man level, but will we see them gain innate abilities that will allow them to do it in a manner that approaches Protection spec levels? Likewise, will we see Protection Warriors who can throw aside the shield and do some real damage? Certainly, if we do (And Blizzard has hinted that that they want all tank specs to be able to do more damage, which would imply they want more flexibility for tanks), we may be able to say that these Death Knight changes led the way.
Of course, it could fail, and we could be left we three talent trees that are thouroughly confused, and a Death Knight class that can be little more than mediocre DPS or mediocre tanking with a few gimmicks. That said, if I can choose to tank or DPS by summoning an undead army and inflicting powerful diseases via the Unholy tree, while my buddy can choose to tank or DPS by freezing his enemies with the soul-chilling power of the Frost tree, and we can both excel at what we do, Blizzard may have hit upon something that will vastly improve the game. Letting tanks play how they want but still be able to contribute what a group needs without all those nasty respec costs can only be good for all those tankless groups that seem to be languishing these days, at the least.