It depends on what level 75 option you took, but Unholy should be able to keep diseases up with Festering Strike/Outbreak. If Unholy Blight isn't saving you from Icy Touch/Plague Strike/Pestilence, then it's not worth anything. Plague Leech on the other hand is giving you a resource rather then saving it, so it's always useable.

Depends on the specific situation you're in.

As a generalization it's:

• Plague Leech - Single target winner
• Unholy Blight - Cleave winner
• Roiling Blood - Massive aoe winner

I imagine many players are interested in how the level 75 talents play out, since the pre-Mists patch hits Tuesday

Here is what we know so far:

Frost uses Runic Empowerment.

• If you're playing 2H Frost, you want Runic Empowerment because it generates the most runes. As 2H frost, all of your runes will be used for Obliterate on single target, so you don't really care which slot RE activates.
  
  
• If you're playing DW Frost, you still want Runic Empowerment. The use is slightly different though. The optimal rotation for DW Frost is spamming Howling Blast, and only using Obliterate to prevent both Unholy Runes from activating. This means you'll be using RE to game Death/Frost Runes.

Unholy can use any talent for comparable single target damage.

• The difference would be in how your rotation functions with each option. Runic Corruption will follow the same pattern it did in Cata, and is the simplest to work with.
  
  
• If you choose Runic Empowerment as Unholy, your focus will be gaming Death/Unholy runes for more Scourge Strikes. RE also has the aoe damage advantage, thanks to Blood Boil being added to Reaping. In the same method that Frost games Death/Frost Runes to spam Howling Blast, Unholy can game Death Runes to spam Blood Boil.
  
  
• If you choose Blood Tap as Unholy, your focus will be turning Blood/Frost Runes into Death for more Scourge Strikes. Blood Tap's main advantage would be the ability to bank Blood charges for key points of the fight. For Mists, it will also help ensure you can Soul Reaper on cool down. This is done by keeping a 5 stack of Blood Charges in reserve, as well as adding more wild card Death Runes into play.

Blood can change the talent depending on the encounter

• The talent that produces the most Death Strikes per minute is Runic Empowerment. However, this is only relevant if the encounter calls for you to spam Death Strike on cool down. For something like Yor'sahj, where there is unavoidable damage being spammed, Runic Empowerment is the best option. RE falls short when you're not using Death Strike on cooldown, and need to time your DS usage. This is because potential procs are wasted, since you can't activate a rune that isn't empty. RE works the same as Cata, where you game Frost/Unholy Runes for procs.
  
  
• Runic Corruption is the talent that allows for the most consistent Death Strike interval, while also allowing you to sit on Death Strikes. All RC does, is speed up your basic rune regeneration rate. As long as you never have both runes in the active state, you're never wasting a RC proc. RC is the best talent for constantly keeping a Death Strike available, without wasting resources.
  
  
• Blood Tap allows Blood to store up regeneration procs, and unload them on Death Strikes at key points. Blood Tap shines on encounters where the damage is predictable, and you can tell exactly when it will happen. Something like the impale on Madness is a good example of this. You know exactly when you're going to get hit for massive damage, and can unload appropriately. The way to use Blood Tap, is to constantly sit between 6-12 Blood Charges. Whenever you hit 11-12 charges, you cast Blood Tap to ensure you don't cap them (12 is the cap). Each Blood Tap requires 5 charges to cast, so you'll need at least 10 charges to unload a full Death Strike. Blood Tap does require fully depleted runes to work, like RE. So you'll have to unload your active Death Strikes, before using Blood Tap.

Blood Tap info

• It works like Runic Empowerment (and Plague Leech) in that it only activates a fully depleted rune. Which means it can't be used unless you have an empty rune slot.
  
  
• Unlike RE, the rune it activates isn't truly random. It has some special logic built into it for each spec, to improve it's use. Basically, it tries to activate each spec's least useful rune and turn it Death.
  • If you're Frost, it prefers depleted Unholy Runes.
  • If you're Unholy, it tries to avoid depleted Unholy Runes.
  • If you're Blood, it prefers depleted Blood Runes.
  
  
• Every Death Coil/Frost Strike/Rune Strike generates 2 Blood Charges, that stack to a maximum of 12. This means you'll want to hold between 6-12 charges of Blood Tap when banking, and always cast Blood Tap when you get 11-12 stacks.
  
  
• Blood Tap is the only talent that gives resources, when Death Coil is used for healing purposes. When you use a Lichborne/Undead heal, or the Glyph of Death Coil, RC/RE will not proc. You will however receive Blood Charges for casting them. The reason this happens, is that you receive Blood Charges on casting Death Coil/Frost Strike/Rune Strike, but RC/RE only proc upon landing the attack. Death Coil healing and Death Barrier don't count as Death Coil landing, so RC/RE can't proc.