Why one tank tree?
Balancing death knights has pretty obviously been a constant battle. There's essentially been six different trees to balance for both PvE and PvP, a tall enough order that almost every major patch has included a large handful of death knight tweaks. Not only did the devs have find a niche for dual wielding, but they had to balance tank talents against PvE talents on the off chance that the interaction would overbalance or underbalance a certain spec in the right combination. They had to squelch diseaseless specs and one-button DPS specs, and Unholy alone took them until Patch 3.3 to get to some semblance of balance.
They already solved the dual wield issue by pushing it squarely into one tree, so it may not be completely unexpected that they're doing the same for the tanking issue. This will, in theory, allow them to focus more on equalizing the tanking classes without having to worry about three separate basic tanking talent setups for death knights. In addition, we should hopefully see a lot more unique tanking toys for death knights instead of two or three per a tree mostly backed up by DPS talents used in creative ways.
Some people have expressed surprise that blood was chosen as the tank tree instead of frost, which is still (inaccurately) seen as the tank tree by many players, but there's a pretty strong set of reasons to go that way. To start with, the developers have spent the last few patches focused heavily on frost buffs and tweaks, but not mainly for tanking. Rather, they've been molding it into the dual-wielding and magic-focused DPS tree. To completely throw all that work out the window would not only negate that work but then necessitate trying to rebalance one of the other two trees to be the dual-wield DPS tree.
That leaves the unholy and blood trees. In this case, they actually come across as very similar later. They both use Death Coil as their rune dump. Gargoyle and Dancing Rune Weapon are almost identical in basic mechanics. They both focus on churning out death runes to use their powerful physical damage based weapon strike. (Yes, Scourge Strike does some shadow damage, but that physical damage component is still huge.) There are smaller differences, of course, such as unholy's extra disease and pet versus blood's superior health stealing and multiple strength boosting talents, but the basic style of the trees has become more and more similar as Wrath progresses. Therefore, turning one into a sole tank tree isn't as much of a loss of a playstyle as it might be, since underneath the hood, unholy and blood are remarkably similar in how they play out for DPS.
The factor that pushes blood over the top is that it has actually been a consistently amazing tank tree for most of the duration of Wrath. While it does fall behind on AoE threat, there's rarely been better for single-target threat (Icy Touch has equalized the threat differential a bit as of late), and blood's superior stamina and healability has also afforded it a top tanking slot in many raids. With all the stamina buffs and self-healing already in the tree, you'll have to move out a lot less DPS stuff. Move Abomination's Strength to frost and you're most of the way there already.
The bottom line
The bottom line is that it is sort of a bummer that we're losing some of our flexibility -- but if handled right, this could lead to more power and more fun for a death knight. Since the developers won't have to juggle six different modes of play crammed into three trees, they may feel more able to flex their legs and their creativity and give us more toys and options without fear of imbalancing the whole thing. Essentially, our breakdown is going to look something like this:
- Two-handed blood tanking, likely continuing blood's focus on high stamina, health drain and healability, but hopefully with more proactive damage prevention ability than it currently has
- Dual wield, magic focus frost, with haste and scads of runic power for its signature style
- Two-handed physical damage unholy, with diseases and pets giving it flavor
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