As for Necrotic Strike, I'm not so sure as to why they changed this ability's cost. I would have to put my money on the fact that it was the "Go-To" strike for PvP, so they wanted to change it up as to make DK's actually use their other strikes more often. Let's face it, we did use the hell out of that when it only costed one unholy.
However, it's now pretty much made every other strike we have in PvP look even more like a joke. Sure, Frost does good damage without using Necrotic, but Unholy? Oh my.... Scourge Strike's physical portion crits plate for around 16k, leading to around an overall strike damage of about 24k. Necrotic crits for around 20k and absorbs quite a bit of healing.
So in other words the cost was changed to break DK's free of the Necrotic dependency, but the buffing of it has made it prove to be quite the contrary.
Instead of making the cost of things change just out of the blue, why not just make an added effect if you wanted people to start using runes for other things?
"Well what do you mean by that?" Well, I'll tell you:
Lets just say instead of changing Necrotic to costing a Death Rune we did things to other abilities that made you want to use them instead when you have your Death Runes up. For instance, Scourge Strike hits like kitten kisses on plate at the moment, so why not make it when you use Scourge Strike with a Death Rune, it deals pure shadow damage hm?
Things like this could be done to a lot of Death Knight abilities, you'd just have to get creative with it. Making Dark Transformation free your pet of snares/roots/CC for a 10 second duration when used with a Death Rune for another example. (Throwing ideas, sue me =P )
"OH well we don't want everyone taking Blood Tap since Death Runes will be at such a high demand then!"
I've got an easy solution: Don't make the 75 talents rune regeneration related. In fact, don't make any talent rune regeneration related. Why not just pick a mechanic and stick with it? Make up some interesting talents to go into that tier and call it a day.
Death Rune costs are lame. I hate it. Death Rune's making things better however, I'm all for. This makes my class feel much more varied and fun to play. Sure, it'd be the same as now managing Death Runes, but they would be a hell of a lot more fun to use.