On the best DPS spec
With the new patch changes, if you're looking for the best PvE DPS spec, it's still unholy. Despite all the nerfs, unholy's still managed to come out on top both because it's just that good and because it relies a little less on RNG.
Frost still has to deal with waiting for procs and then lining them up to be used perfectly with the right abilities, which can take a little more finesse to manage properly. That said, there is still some good news for frost lovers in that a perfectly played frost spec is in theory less than 5% behind unholy -- that is, still enough that if you probably want to take unholy if you want the best of the best, but almost to the point that you might not be laughed out of a raid for taking frost. In addition, 2H frost is now nearly on par (less than 1% behind, at worse) with dual wield frost, so it does honestly appear to come down to a preference of which type of weapon you want, gear weights aside.
Finally, dual wield unholy is dead as a doornail and likely to stay that way, thanks in part to a bunch of weapon strike buffs and the changing of Sudden Doom to proc off the main hand only.
Unholy, of course, has a completely new mastery called Dreadblade. It's essentially a shadow version of Frozen Heart, and since we use shadow damage as unholy almost more than we use frost damage as frost, as you might have guessed, it does affect how we gear. Now, haste is still the superior stat. This is primarily because it affects our ghoul in a way mastery won't and because we do still have enough leeway with Unholy Presence to fit in more attacks before hitting the GCD cap. So, as before, haste is easily the secondary stat winner after you've hit the 8% hit cap. That said, mastery now pretty solidly blows away both expertise and critical strike, and unholy death knights should now find it more useful to take haste/mastery gear or to reforge critical rating into mastery on haste/crit gear.
The mastery change has also affected how frost death knights play. 2H frost disciples will find that their stat weights are a lot more similar to unholy as not. This means that it should be a lot easier for up-and-coming death knights to switch from the awesome leveling spec that is 2H frost to the more superior 2H unholy raiding spec. It also means that you can more easily keep 2H frost as a dungeon or solo farming spec or even an AE raid spec, while keeping unholy as your single-target/raid boss spec. It's a relatively minor thing, but it is nice and convenient.
The other way that the mastery change affects frost is in the talent Virulence. Virulence is essentially the old unholy mastery in the form of three talent points with no additional scaling. As a mastery, it was severely underpowered. As three talent points, it's surprisingly worth it. Unholy, of course, will take this without hesitation, but frost will probably want to do this too, even if it means giving up Improved Blood Tap. Sure, IBT is very useful for getting off a timely Pillar of Frost, but it's still one more complicated button to push in a spec that already depends quite a bit on pressing the right button at the right time. Virulence, however, is a nice, awesome passive boost to your DPS, and one that so far seems to parse out to more damage than IBT for your average death knight.
One final note should be taken in reference to Virulence's old function, that of providing 9% spell hit. Since that 9% is now innate to every death knight, 8% hit essentially becomes the "hard cap" for 2H-wielding death knights. Any hit we get past 8% is essentially useless, since that innate 9% is covering the gap between our spell hit and our melee hit. So in essence, don't be afraid to reforge or regem away extra hit rating.
Tanking: The mastery vs. avoidance debate continues
Honestly, even after patch 4.0.6, there's still no clear victor in the mastery vs. avoidance debate. Mastery actually got an overall buff for blood tanks this patch, since Blood Shields now stack. In essence, you can throw out two Death Strikes back to back without having to deal with one overwriting the other. Of course, in practice, they're stripped off pretty quick anyway, but it's still something to watch out for.
One new theory that's seen some traction is the idea of going back to the old unholy method of stacking avoidance to keep Bone Shield up as long as possible. The argument is that if you have high avoidance, your Blood Shield will stay up longer, since multiple Death Strikes will keep it refreshed, and it will only be depleted on those instances where a hit gets through your other defenses. Right now, I've seen only anecdotal evidence for this new theory of gearing, but it at least bears some mentioning.
Overall, though, much of what I wrote in my previous column on the issue remains essentially true, and you'll continue to have to find your own balance until the theorycrafters can agree on some real numbers -- or tier 12 content throws another level of mudflation into the mix and muddles up the stat weights all over again.
Follow our road map to leveling your DK from 80 to 85, and then learn the ropes of endgame play with WoW Insider's DK 101 guide. Gear up with pre-heroic gear for DPS DKs and keep track of what's happening on your hotbars with our guide to DK spell alerts.