The thing I hate about necrosis, is that it only really boosts white damage, which is unappealing to me for two main (PvE and PvP) reasons:
1) Auto attacking things to death isn't fun (which is why I personally hated the retadin playstyle prior to WotLK), and talents that boost that type of mechanic I look down upon. Mechanics that exist simply to boost white damage hold very little appeal to any DK not planning on dual wielding. This is directly contradictory to staple, spec-defining talents that you get later in the tree, such as scourge strike, which screams 2hander.
2) In PvP, talents that focus mainly on boosting auto attack damage aren't highly sought after at all, unless those same talents also boost instant strike damage as well. A good example would be dual wield spec for rogues. By itself, it's fairly unappealing to rogues that care about what matters in pvp: Burst damage. Where it DOES become appealing, is how it affects mutilate burst damage. That extra burst damage coming from your offhand dagger on this big crit mutilates is a huge attraction for pvpers. Necrosis does nothing of the sort. It's a small, flat boost of white damage. That's it. It's an interesting concept, but doesn't really change the way a deathknight will use his or her active abilities, and thus I consider it to be a rather boring talent.
To along with this, look at a different example of how this affected another class:
BM hunters were, by far, the most damaging spec they could be: In PvE. Why? Because a lot of their damage was off GCD via the pet, and they had a very balanced shot rotation via steady-auto rotations. However, to do this sort of rotation, you essentially had to sacrifice ALL mobility, as steady shot is a "cast time" ability, meaning you can't move while you use it.
So, what was the supreme "DPS" spec in pve, failed miserably in Competitive PvP (arena), turned out to be pretty bad when mobility and burst is suddenly king. Marks was their best bet, since it relied heavily on front-loaded damage, and you could reposition yourself between front-loaded burst cooldowns, or use that time to drop traps, wingclip and reposition, etc.
Now, given that context, I also feel that our "pvp presence" of unholy presence fails to make up for the loss of damage gained in blood presence, since Auto attack damage is NOT an acceptable replacement for 15% overall damage lost. You could argue that the extra 7% movement speed of being in unholy presence vs having some sort of run speed enchant could somehow theoretically make up for whatever the increased auto attack damage doesn't cover, but I doubt it. Though, I cannot prove this point at this time.
Necrosis falls in the same category, and will probably end up being regulated into a pure pve spec, and probably only for dual-wield specs.