My Opinions of the changes you mention.
Grit - Parry Bonus, This will increase the tankiness of DRK, but it might be a little overpowered unless they nerf another aspect of DRK. DRK and PLD right now is very similar in terms of mitigation except the shield, if they gave DRK a static Parry Bonus and put them on par with Paladin, people won't use Paladins anymore since their Damage is so much lower than DRK and WAR, this is the reason why WAR didn't get a parry bonus to their tanking stance. I mean if DRK has the DPS of a WAR and Tanking ability of PLD, why use the other two right?
Reprisal - I agree that its really stupid that you can't keep the debuff up 100% of the time, but putting it on a 15 sec recast is a little too overpowered. A 210 Potency attack off global cool down every 15 seconds? I suggest they just increase the duration of the debuff to like 35 sec or put it on the global cool down (like paladins version) to make it more reasonable. Or they will have to make PLD's version off GCD and give WAR a similar off GCD counter attack.
Dark Dance - Reasonable change.