Shifter is interesting because it allows you to take the chassis of a spellcaster (no joke; instead of having decent defenses, HP, and recoveries like the Cleric, the Druid is at Wizard/Sorcerer level) and turn it into a melee fighter. The only intrinsic bonus you get is the fact that Beast Form Attack offers really great damage per round (DPR). So you're basically a glass cannon, particularly at low levels. But that's where Aspects come in. Aspects are limited-use (recharge for Adepts) and give you various mechanical bonuses while in beast form to model the differences between various animal forms. Bear form makes you tougher and lets you mow through mooks, leopard form makes you quick and opportunistic, etc. The key is that almost every form provides access to a stackable bonus to AC and PD (and sometimes MD). Pop one aspect and now you've at least got Bard/Rogue level AC. Take the feat(s) that let you stack aspects and you can even be somewhat tanky with the right ones. While this was my hunch upon reading them, play experience has confirmed that a defense bonus is pretty much a necessity when designing Aspects. Anything you transform into should have baseline toughness better than a spellcaster.
The existing Aspects are all really cool. There were a couple that I initially considered underpowered and didn't think I'd take, but on a whim I used them with my character and found them to be really fun. There's a nice mix of standard beasts like bears, tigers, and wolverines as well as more magical stuff like giant mantises, owlbears, and behemoths. The one glaring hole is the lack of the obvious Wolf Aspect, and mechanically there's a paucity of control and mobility. Basically, the published Aspects are very focused on boosting raw damage, durability, or both. I figured I'd fix that by making Wolf Aspect more control-heavy (as well as making natural odd rolls a little more exciting). It naturally rewards "pack tactics" as well. I imagine it would be quite fun to combine with Animal Companion Initiate (sidebar: while Shifter Adept is really cool, I'm convinced that Shifter Initiate is the weakest of the initiate options, to the point where it's probably not worth taking). As a counterpoint to Wolf Aspect I've added another magical beast, the Blink Dog (mostly because they're underrated). This one's very focused on both mobility and defenses, making it the ideal "striker" Aspect. It'll be quite effective at getting behind enemy lines to the priority target(s) and dealing with them. It can also zip around the battlefield giving it a similar feel to a Monk or Rogue (or a 4E Predator Druid).
After analyzing the Druid a bit more in-depth, comparing it to other classes, and some discussion online I've decided that a "Natural Armor" feat isn't quite what the class needs. In fact, I think a reversion to the playtest Druid's base defensive stats is in order. Thus, I've decided on the following suggested house rule to bring the 13th Age Druid up to consistency with tradition as a physically tougher spellcaster, if not one that's armored as well as a Cleric.
House Rule Revisions to Base Class
Revised Druid Armor Table
Type Base AC Attack Penalty
None 10 -
Light 12* -
Heavy 14 -2
Shield +1 -2*
Revised Druid Hit Points
Change from 6 + Con mod to 7 + Con mod.
Revisions to Warrior Druid
Your AC in light armor is 14 instead of 12 like most other Druids.
Your base hit points are 8 + Con mod instead of 7 + Con mod.
New Shifter Aspects
Initiate Effect: Gain a +2 bonus to PD. If the target is engaged with one of your allies, your natural odd beast form attacks deal an extra die of damage.
Adept Effect: As initiate effect, plus you can choose to make the target of your natural odd beast form attacks Vulnerable or Hampered. Also, the bonus to PD applies to AC as well.
A: Allies gain a +2 bonus to melee attacks against enemies engaged with you that you hit on your previous turn.
C: Once per battle you can make the target of a natural even beast form attack Vulnerable or Hampered.
E: Until the first time it recharges each day, Wolf Aspect is Recharge 11+ for Adepts instead of Recharge 16+.
Blink Dog Aspect
Initiate Effect: Gain a +5 bonus to Disengage checks and when you hit with a natural 18+ the target is Dazed until the end of your next turn.
Adept Effect: Gain a +2 bonus to AC and PD and once per battle you can teleport anywhere nearby as a free action.
A: Gain a +2 bonus to beast form attack if you moved to engage the target this turn.
C: Once per battle roll a save when you're hit with an attack. On a success you take only half damage.
E: Until the first time it recharges each day, Blink Dog Aspect is Recharge 11+ for Adepts instead of Recharge 16+.