I've been playing as both MM and Survival, in both PvE and PvP, and I've found serious flaws in Survival's defensive skills. These may not be so apparent in PvE (thought they do have a terrible effect on our DPS if we need to mitigate damage), but they are very clear in PvP.
Currently, Survival has 5 main defensive skills: Exhilaration, Survivalist, Aspect of the Turtle, Aspect of the Cheetah and the artifact trait Bird of Prey. Five different abilities to defend yourself may seem excessive, but they have poor synergy between themselves and with the spec's offensive traits.
Exhilaration heals the Hunter for 30% max health, and heals the pet for 100% with a 2-minute CD. By itself it's not a bad ability, if the other skills worked better, Exhilaration would not need any buffs.
I do have a problem with the Artifact trait Hunter's Bounty, thought. It reduces Exhilaration's CD every time you kill an enemy. This is mostly a leveling/soloing perk, with almost no effect in Raids or PvP (in which it's very rare for you to get the killing blow). I'd suggest changing it to "when a target afflicted with your Lacerate dies". This would allow the Hunter to benefit more from this perk.
Survivalist is a passive that heals 15% of the Hunter's (and pet's) health over 10 seconds when you kill an enemy. It's a nice leveling/soloing perk, but has almost no effect in PvP or raiding, just like Hunter's Bounty. I'm listing it here because I'll reference it later, but it needs no changes.
Aspect of the Turtle is the main problem in Survival's defenses. It has a 8 second duration, reduces damage taken by 30% and makes you evade all attacks, with a 3-minute CD. However, you can't attack while using it. The big problem is that this was designed as a defense for a ranged class. It's meant to allow the Hunter to run away or survive long enough to get help. Survival is a melee spec, with no Disengage or other tools to get away from an enemy. In essence, this Aspect of the Turtle just makes the fight longer by 8s, since you can't do much while under it. Yes, you can place traps, but that's it.
Compare it to most melees' defenses, and you'll see that Survival can't outlast any melee spec. If the enemy uses defensive skills, your damage is mitigated while you take full damage from the target. If they use offensive skills, you can't mitigate damage without stop DPS'ing the target. Either way, you almost aways die faster than your opponent.
It's even worse against enemies that can heal. Aspect of the Turtle protects THEM from you, allowing them to cast two or three healing spells before you can attack again (and you can't even interrupt the spells!).
Aspect of the Turtle could be somewhat better, but still flawed, if we had the BM/MM's artifact trait that allows the Hunter to heal while under it. At least we would get out of the Aspect with more health.
My suggestion is to change it to something like an "Aspect of the Gorilla" (or any other big predator, really): 30% damage reduction, no evasion, but allows us to attack. Make it 2-minute CD with a 5s duration so it can be used more often. Either that, or allow us to attack while under Aspect of the Turtle, even if we cause reduced damage (like Paladin's Divine Shield).
Aspect of the Cheetah needs tweeks. Harpoon is nice for mobility, but sometimes we need to move somewhere there's no enemy. Also, remember that without the Throwing Axes talent we have no ranged ability whatsoever, so we need a way to reach enemies if we are slowed or immobilized just after a Harpoon (which happens often against ranged players). The 3-minute CD is just too long, even with the Artifact trait that allows it to be reduced to a 2.4 minute CD coupled with the PvP talent that reduces it by half (1.2 minute, or 72 seconds). Its standard CD should be halved to 90 seconds, so the trait reduces it to 72 seconds, and the PvP talent makes it a 36s CD.
Finally, the Artifact trait Bird of Prey needs serious buffs. It allows our Raptor Strikes to heal us for up to 30% of the damage caused. Problem is, Raptor Strike does very little damage, and thus heal almost nothing (barely more than 1% of our health each time it's used). Even if the heal was meaningful (like 100% of the damage), it would come at cost of our DPS. Under its current iteration, Bird of Prey is useless: any enemy will still outdamage your heal while in turn you are sacrificing your DPS.
My suggestion is to either buff really hard the trait (like upping it from 10/20/30% heal to 33/66/100% heal) or to change it to, instead of healing based on damage, make it a chance to activate Survivalist (like a 15/30/45% chance to proc it on each Raptor Strike).
Finally, we have no way to defend ourselves while under CC. As such, once we are stunned, we pretty much will lose the battle. With the suggestions above, maybe no further change would be needed in this regard. However, if no other changes are made to Surv's defenses, then at least allow us to activate Aspect of the Turtle while under CC, please.
My suggestions are not perfect, and taken together may even make the class overpowered, but it's undeniable that Survival needs some help in its defensive skills. Please, I ask Blizzard to take a look at these concerns.
Personally, I think Mongoose Fury should give you 5% damage reduction per stack. That makes it safer to stay on your opponent at higher stacks, but it's not too unbalanced since it's stacking over the course of 14 seconds.
Next, I think Survivalist should trigger off Turtle, Feign Death, and Freezing Trap.