Part of the problem is fixing this is that the issue inherent to it are all tangled up in point 2. Blood Tap is overpowered because we need death runes more than ever. But even if that weren't true, there would be balance issues between the three.
At this point, I think tier 75 just plain needs to go. Redesign it. Get something new in there. You could even move the level 90 tier down to level 75 and get some new damage talents for level 90. Make Runic Corruption baseline for the class. This removes the awkwardness of rune tetris and offers some much needed relief for our increasingly complicated playstyle. If you take care of point 2, you can even give us back old style Blood Tap, too.
2. Reevaluate unusual rune usage. After Wrath, Blizzard started to get a little bit creative with rune usage -- namely, it put Scourge Strike, which was once a traditional frost/unholy rune spender, on unholy rules. This of course did not at all fit with other traditional strikes such as Blood Strike, Plague Strike, and Icy Touch. Blizzard tried to fix this by adding Festering Strike, which took up the blood and frost runes now left orphaned by Scourge Strike.
Unfortunately, this has still left unholy in an ultimately awkward position where solid base class skills like Icy Touch and Plague Strike and extra lifesaving and utility buttons like Death Strike suddenly just throw everything off in a way they don't for frost or blood death knights. In addition, it makes it a lot harder to transition from frost DPS to unholy DPS when you have to learn a whole new way of spending runes. This is not to say unholy should be functionally identical to frost, but there should be some continuity and some ability to communicate with other class abilities. In short, I truly believe Scourge Strike should be returned to an frost/unholy rune-spending heavy hitter.
Of course, this issue is relatively minor compared to the real can of worms opened in Mists, that we now have abilities that use death runes directly, most notably Soul Reaper. In Wrath and Cataclysm, death runes have been a mostly intuitive way of adding a few more of your signature strike into your rotation. In Mists of Pandaria, we will specifically have to set aside these death runes to maximize our damage. This becomes most notable when Soul Reaper starts giving bonuses. You'll essentially be changing your entire priority and rotation to generate as many death runes as possible. This ties back into our first point, as this makes Blood Tap immediately the best choice for DPS simply so they can generate death runes more or less "on command."
Death knights already juggle so many indicators and resource types. Adding death runes as a resource in their own right rather than a side effect of the rune system is going to add an extra level of complication we do not need as a class. It is likely too late in the beta for the developers to simply scrap Soul Reaper and give us a new level 87 skill (though like I said last week, I'd love for them to do it and give us Gorefiend's Grasp as a base level 87 ability).
But I do predict that the use of death runes is going to be a prime source of confusion, burnout, and low DPS among death knights in Mists of Pandaria. Our rotations are just too complex for their own good anymore, and death runes are going to be one of the prime causes. Abilities that directly use death runes and rotations that can't support basic class skills need to go.
3. Address those nagging DPS spec balance issues. This point really boils down to the existence of Unholy Might and the balance of two-handed and dual wield frost DPS. There are other issues of varying importance with the specs, but these two have really haunted the class all through Cataclysm and look like they'll stick around for Mists, as it currently stands.
Two-handed frost DPS has been on an unfortunate roller coaster through much of Cataclysm. It fills a much-needed DPS role in the class, offering the flavor of the heavy hitting two-handed weapon playstyle that blood DPS used to fill. Unfortunately, it hasn't always done that successfully. Dual wielding continues to offer more weapon damage than two-handed frost. While Might of the Frozen Wastes sometimes managed to fill the gap, it was generally nerfed back down pretty quickly. As a result, dual wielding has been the clear choice for frost if you want to min-max your damage for most of Cataclysm. It's difficult to say if this will continue into Mists of Pandaria, but right now, dual wielding continues to pull away from the pack pretty handily.
Unholy Might's issue is scaling. Since so much of unholy's damage comes from things like pets and diseases, Unholy Might looks at first blush to be perfect for that reason alone. It affects all of unholy's damage, making everything nice and uniform. Unfortunately, what it doesn't do is affect weapon damage nearly as much as it should. As a result, weapons upgrades are often lackluster as compared to other melee DPS specs. In addition, Unholy Might has become subject to many patches and hotfixes, as Blizzard has used adjusting it as an apparently quick fix to unholy DPS levels.
Now, in the long run, this is probably the least urgent thing I've discussed over the past two weeks. In some ways, one could argue this is part of the flavor of the specs. Still, Unholy Might and Might of the Frozen Wastes are perennial thorns in our sides, being buffed and nerfed repeatedly when most of the death knight community feels the right move would be to go at the underlying issues they highlight with their specs.
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