The first thing that strikes me is that the text for Aspect of Nature is different from the table; the table has stage one at 3-6 (not 4-7), stage two at 6-9 (not 7-10), stage three at 9-12 (not 10-13), stage 4 at 13, 15, and 17 (not 13-15), and stage five at 18 and 20 (not 15-17).
Gods, I'm a terrible editor
I think the text ones were from before the aspects of elements.
Stage Two Nature: Do you need to specify +5 feet for the legs of the deer one, or is just +5 customary?
Added '. Generally, there's nothing else it could be, but specification is not a bad thing.
Stage Three Nature: How long does it take to spin a spider's web?
The entire action is an attack action. It works like web spinning monstrous spiders. I will specify more information and clarify the link.
Is it necessary to state which natural weapons are primary (and get full strength) and which are secondary, or is that inherent in their type?
"When a creature has more than one natural weapon, one of them (or sometimes a pair or set of them) is the primary weapon. All the creature’s remaining natural weapons are secondary.
The primary weapon is given in the creature’s Attack entry, and the primary weapon or weapons is given first in the creature’s Full Attack entry.
A creature’s primary natural weapon is its most effective natural attack, usually by virtue of the creature’s physiology, training, or innate talent with the weapon."
Emphasis mine. The best natural attack you have is your primary if there's nothing specifying otherwise.
Stage Four Nature: What is the effect of the wolverine's rage if the rage ability the Druid already has comes from a template, spell, item, or other source that does not specify an effective class level for the rage?
He gains the rage of a 1st level Barbarian, since he does not have anything rage related that a +1 class level would determine.
Stage Five Nature: When curled up like a hedgehog, must the Druid take the Full Defense action each turn, unless they choose to uncurl? Or is this a "free"(-action) +10 AC and 5d6 retaliation for not being able to move?
"and is considered to be taking the total defense action"
Capitalizing for emphasis though.
Stage One Elements: Wrapped in Fire, and its electric and negative-energy counterparts, seem powerful (at least in comparison to the rest). They deal 6d6 damage (no save), plus 3d6 (at least) and action denial from fire, and the electric or negative-energy counterparts don't have a phrase preventing the Druid from walking back and forth, crossing the 10' line to deal 6d6 damage multiple times. Additionally, does the Acid-breath one count as a breath weapon?
None of them possess such a phrase preventing multiple hits. Adding one in. Fire doesn't deny actions though, unless you want to end it early.
And I would say that being able to use your breath as a weapon counts as a breath weapon, yes.
Stage Two Elements: Do the movement-speed granting Drifts overwrite your speed (such as the Nature-aspect fly speed, or a racial Swim speed), or use the faster speed? (Again, this is a question I feel that more experienced people would know, but it's unclear to me.)
They use the speed listed. Twice your land speed for swim, same as land speed for flight. I'm not sure I understand the question, though? They are different movement modes. The same as any creature that has a flight, swim, or climb speed.
Also, the healing-from-damage by absorbing elements one seems to they have very high heal-to-damage ratios; even making half of your save means that you'll gain the full spell's value in HP. (Taking half, and healing 1.5, means that you heal the full damage of the spell, despite only making your save for "half" damage.) From memory, I feel like I've seen that kind of healing at three damage = one healing, but I'm not sure.
I had remembered the same ratio, just had it backward. But you're right, saving even against a partial affect would give you a net gain of hit points. Reversed the ratio.
Druidic Avenger: I understand how it seems to work, though by giving Druid rage equal to a Barbarian of its level (even without Mighty/ Greater Rage), I feel that it makes the Barbarian superfluous; there aren't that many other substantial features the Barbarian gets, other than Rage.
This gets into a little murkier territory, as its meant to work wit the Barbarian variant from the same setting. It has a much expanded rage ability (including new types and larger buffs at the higher steps), as well as other abilities that compliment it.
The Avenger variant here is pretty much just how the unearthed arcana avenger variant interacts with the barbarian changes.
Coyote Druid Racial Substitutions: Fits, although I feel that the substitution levels don't make any sense for an Avenger, which bugs me because Avengers get special bonuses for being a Coyote and giving up the racial Dog companion.
That's kind of the point though. The Avenger wouldn't be able to get the cap on the generic Druid sub levels, since they don't qualify for the cap if they give up their animal companion.
Also, the level-9 row on the table is missing the Stage 3 Aspect.
Fixed.
Elemental Affinity: This scares me, because of the damage problems it could cause, thanks to the auto-damaging Elemental Drifts. However, it's only available at level 18 (you gain Aspect of Elements at level 16), so that reassures me... a little.
Those specifically would get a bit powerful, yes. But, especially at that level, there are generally better things for spellcasters to be doing than using feats to focus on direct damage.
Affinity Master: Again, damage. Scary. However, there's no way to gain this feat without extra bonus feats, or going Epic. This is thanks to the fact that Elemental Affinity can only be taken at level 18, and you don't get another feat normally until level 21, which is Epic. And honestly, there are better things for Epic feats.
Share Aspects: oh god the damage[/i]. If you, say, sell your soul (Pact Certain) at level 18 (shortly after getting Elemental Affinity) to get Affinity Master and picked up Share Aspects earlier, you've got (6d6 (fire) + 12d6 (electricity) + 12d6 (negative))*2= 60d6 of damage every time you and your Animal Companion get close to something. No save. Oh, and if you have the movement to back up and then back into the 10' range? Another 48d6. Per 10' of extra movement you have. This... scares me.
Just throwing on the not causing damage twice clause to those abilities fixes this. But, you're right, if you dumped everything into nothing else, you could do some serious AoE damage passively. Think I'll drop it down to 4d6 for the auras.