Nurturing Instinct's benefits are threefold; a). increasing feral survivability in PvP, b). increasing the efficiency of heals on yourself or others while in feral gear for PvE, and c). increasing survivability in raids or 5-mans where you, as melee dps, are among those most likely to take damage besides the tank(s). So, let's look at these one by one:
A). Increasing feral survivability in PvP
Feral currently has a number of problems in PvP at 70. The example of such players as Deep and Azgaz aside, it's sometimes tough to argue that resto (and sometimes balance) isn't the way to go. There's the always-fun Cat form range bug to contend with, the sheer squishiness of Cat form itself, the nerf to bear armor, health, and damage, and the constant danger of being stunned or incapacitated outside of forms. It's especially regrettable that the two Druid dps buffs, Leader of the Pack and Moonkin Aura, give 5% crit to yourself and your party when both the likelihood of and damage done by a crit are reduced by resilience.
One of the things you will tend to see over and over again in good feral PvP videos is constant healing. Heal early. Heal often. Heal as if your life depended on it, and most of the time it does. Pre-emptive healing is often the key to surviving when your fighting style consists of being a Rogue without Sap, Blind, or Vanish, a Warrior without Mortal Strike, Hamstring, or Overpower, or a Mage without Polymorph, Blink, Frost Nova, or Ice Block. Your tactic as a Druid is to outlast your opponent, because, assuming equal levels of gear, you're unlikely to do more damage than they can. When your healing efficiency is improved significantly in feral gear, that's a major advantage as long as you've got the mana to keep shifting in and out of forms to toss HoT's. Although, with that said, one of the enduring ironies of feral PvP is almost certainly that the two maxims to be obeyed - don't go OOM, and never stop healing - are two goals very much at odds with each other.
Itemization-wise, most of the gear with a ton of Agility on it is also Rogue gear without +int - so keep saving honor and/or arena points for the Gladiator's and Vindicator's Dragonhide gear. Your mana pool will thank you, and with the amount of Agility on high-end Druid PvP gear, this might have a significant effect on traditionally poor feral mana efficiency.
B). Increasing the efficiency of heals on yourself or others while in feral gear for PvE
PvE-wise I would argue that the change to Nurturing Instinct is effectively more business as usual. Nurturing Instinct has not been a popular talent for most PvE builds if only because the cookie-cutter 0/47/14 tanking/cat dps build is so streamlined. This is the build that will squeeze every last little bit of mitigation, threat generation, and cat damage from whatever gear you manage to get your hands on, and I'm hard-pressed to pick anything in there that I would sacrifice for the new version of NI. I would nominate Intensity as the most likely candidate for the average feral, or Thick Hide if you're one of the lucky, lucky souls in full Tier 6 with more armor in bear form than you know what to do with (although sacrificing Thick Hide will require you to take Brutal Impact to reach the third tier of feral talents, BI is another useful PvP talent so it will complement NI in that respect very well). The problem is, Intensity is one of the talents that makes it possible for a feral Druid to heal effectively while in resto gear (and its secondary effect will generate 10 rage instantly from the Enrage ability in bear, which I have found to be useful, though not absolutely necessary, while tanking), so you're trading significant mp/5 while casting for better healing efficiency while not in resto gear.
If you stay feral absolutely all the time, hate healing, hate being resto, and don't ever plan on turning resto - Nurturing Instinct is worth your time, and you probably won't miss the insta-rage from Intensity too badly (although, if you hate healing that much, do you still promise to toss heals on other players with your newly-efficient, Agility-enhanced HoT's?). If, like me, you enjoy throwing on healing gear for a BG or the odd 5-man for your friend's alt when they need a healer, NI is a potentially useful talent that you may or may not have the room for; if you're in resto gear to help heal something, you'll get very little benefit from gear with no Agility on it. That being said, I'm still likely to give it a go for a week after the patch hits, if for no other reason than -
C). Increasing survivability in raids or 5-mans where you, as melee dps, are among those most likely to take damage besides the tank(s).
The secondary component to the newly-reworked talent is that the healing done to you will rise 10/20% of your AP while in cat form. As stated earlier, this can be significant, especially given how high cat AP tends to be in relation to other physical dps classes (albeit we don't scale anywhere near as well), and is all the more useful when you think about the job your healers are going to have in your run of the mill Outland heroic or raid.
There is a great deal of additional healing required in today's 5-man's and raids, and I'm not the first person to note how melee-unfriendly many encounters are:
I'm very keen to see how much easier it is to keep my furry little rump alive in a raid when I'm not tanking, and because a feral's job is so frequently to tank and then switch to dps, Nurturing Instinct might very well become a must-have talent for the raiding feral. I'm reserving judgment until I get the chance to see the talent in action once 2.4 hits; it's entirely possible that the talent's only worthwhile on progression content, which gives it a fairly fixed lifespan of use. I'm in agony over which talent I'll have to drop in order to accommodate it. Intensity, we hardly knew ye.
Ultimately Nurturing Instinct is likely to benefit feral PvP builds the most, or just provide the added convenience of better HoT's while grinding. With feral being so poor for arena currently, it remains to be seen just how much NI is going to improve matters. One of the miserable truths of feral PvP is that the majority of Druid PvP talents are pretty incompatible with the talents you need to maximize (and, in the case of Survival of the Fittest
, ensure, due to the lack of +defense on leather gear) PvE gameplay. While this is true of most classes, hybrids have it particularly tough in that the spread of PvP talents amongst all three trees can hit their effectiveness outside of BG's and arena very badly. We'll see what happens.