Hmm... By the book, I don't see that Unnatural Characteristic will solve Wolf's fear problem. In spite of the name, Unnatural Characteristic doesn't double the Characteristic, only the bonus that comes out of the ten's column of the characteristic. Thus, if Wicke were to gain Unnatural Willpower, her Willpower would still be 40, and fear tests would still be rolled on 40 or less, but if a psychic power added the her willpower bonus to its damage, the damage bonus would go up from 4 to 8.
Also, I sympathize with the conservative use of Traits. Traits "may be gained through other means later in life, but they are almost never gained through choice." One character choosing a Trait would be... against The Way. Half the party choosing the same Trait... I don't think so. I would support Rob if he decides that Traits are reserved for demonic pacts, characters that have just burned fate points coming back from the dead, failures on corruption tests, characters being bitten by a radioactive spider, or something similar. If a character is too bland and boring (not our current problem, I think) and both player and game master both want to spice the character up some, sure, the player might encourage the game master to arrange for an exotic transformation. Still, I wouldn't want to see traits used casually to change game balance or to maximize power gaming ability.
I personally think the perils of the warp mechanisms are too strong. Wicke currently has more psychic powers she has never used (six) than those she has used (three). For me, casual use of psionics has never been worth it, so Wicke generally avoids using psi, but Rob has not relaxed the psi rules from book. I'd rather play Wicke as a laspistol and sword nobody than risk the dice transforming her into a character I won't want to play.
The fear mechanism might also arguably be too strong. Still, Dark Heresy isn't D&D. A mechanism that forces many characters to fall apart when confronted with the unknowable is appropriate to the 40K universe. I can see why Rob won't want half the party bypassing such a mechanism. I also don't see him as over using fear, though if the player's front line fighters continue to maximize their offensive hitting power, he might have to start using mechanisms other than lots of bad guys with swords and guns to present us with reasonable challenges.
Anyway, I'd again suggest folks look at the Unshakable Faith and Fearless talents. These would boost bravery through the 'Elite Advances' mechanism, which might be less drastic a thing than letting players select Traits. Would it be such an awful thing to let a feral world character take Unshakable Faith out of sequence, or perhaps a psycher take Favored by the Warp a bit early?
By the way, Rob, it seems that Psychers can take Unshakable Faith at Third Level. I have 100 XP in hand, and vomit all over the front of Wicke's space suit... Very undignified. Hmm... Re-rolling fear tests with a 40 Willpower. 64% chance of passing. A chance to show up that cowardly wimp, Wolf...