The biggest pieces of news so far, on the heels of 3.3.3 is some Blizzard chatter on the new Mastery stat and talent tree changes in Cataclysm. Mastery is a new stat that Blizzard is introducing to replace redundant and inconsequential stats which make itemization and gearing more difficult than it should be. As a Death Knight, you will no longer see stats such as Attack Power, Armor Pen, or Defense on your gear. These itemization points will be devoted toward things like Strength, Stamina, Haste, Crit, and Mastery.

Blizzard is using this opportunity to remove talents that are not dynamic; talents that give passive bonuses or are really not that interesting. In exchange for these changes, you will gain Mastery with that given tree as you spend more points in it. To give an example, Blizzard used Frost for Death Knights, they said that some Mastery bonuses for this tree could include increased Damage, Haste, and Runic Power generation. The amount of the bonuses you gain would depend on how many points you’ve spent in that tree.

I’m fairly excited about this. I started to get annoyed with item stats back in Zul’Gurub when Blizzard started adding hit rating (or at the time hit %) to gear. After hit came haste, and armor pen, and these other stats. This has had two unfortunate results; It’s harder to understand gearing for people who aren’t experts and It’s forced Blizzard to make a greater variety of gear sets to suit all these different play styles; This, I’m sure, has made dungeon itemization a nightmare for the designers.

While I see the potential benefits of this plan, it has several major issues, especially for Death Knights. I’m sure maybe that I don’t understand everything, because all has not yet been revealed, however at this moment I see this potential pitfall. Eyonix mentioned that Mastery bonuses would be in line with that spec’s specific role. However, the Death Knight trees are built so that no one particular tree is for tanking, dps, or pvp. Out of all the classes, Death Knights idealize the take the player not the class (or in this case talent tree). They expect all the trees to do everything, so I don’t see how you can apply this same Mastery philosophy to Death Knights as you would to a Druid or Paladin, whose trees each have specific roles in mind. To answer this very concern, Eyonix stated the following:

I hate the fact that I need to be extremely vague here, but there is specific information that we're not quite ready to discuss that will alleviate your concerns in this area.

We have quite a bit of information coming regarding the next expansion, so just hang tight, and have faith. I know many players will naturally be skeptical of what I'm about to say...

I've honestly never been more excited about this game with what's to come (and I've been playing for over 8-years), than I am now. I cannot wait to share more information with you, and I can't wait for you all to experience it first hand.

And to be honest, I can't wait for me to experience it first hand!

  -  Eyonix ( Source )

Required Reading

    Mastery System Preview

    Cataclysm Stat & System Changes