I find the traits to be very varied in their attractiveness. In order of appearance...
Toughness: This one owns, hands down. More health is more healing received, more sturdiness, and so on, and so forth. This is partly due to the fact that all heals are %-based, else it might just pay off to try to take less damage. I am going for this trait on all my characters. The entire party. Requiring 20 strength on top of the 50 constitution, I don't mind, it's increased threshold.
Willpower: Okay no. As the game goes on, there's certainly a need for larger health pools, because enemies do more damage. There isn't a need for (much) larger mana pools, because your spells don't cost more as you gain levels. Especially if you have % regen effects, there is a point where you couldn't run out of mana even if you tried. My level 16 druid, with 20 will, has 180 mana. That's 18 mana/turn from Vigorous Sonata, plus another 6 from her necklace, so 24/turn. Her most expensive spells cost 30 mana, so if she used those every single turn, she would run out of mana after 30 turns. That really isn't a concern. Now if she had 30 more will, that'd be 330 mana - already 33/turn. And then +15%, um yay?

Sure you could say, but what if Vigorous Sonata gets nerfed or you want to use another song or you just don't always have Jariel? I'd still fix the mana issues with regen items, not an absolutely gargantuan mana pool. And let's keep in mind that each of the 50-20 traits is a huge commitment, requiring 15-20 levels worth of attribute points (with NM point gain). They're not something you pick up casually. Especially the 40-50 step requires 30 points.
Dual Wield: I don't like this one either. Who is going to take it? Chalassa, whose skills use her bow anyway? Yeah no... hunters? Rangers, Krimm or Vaelis? The thing is, if you're going to rely on your melee weapons, you're obviously gonna be in the frontline. And sure, in that regard, it makes sense to increase your skill. My frontline characters would see a ~25% defense increase if they spammed their skill up that much... but at what cost? Obviously no constitution spam, and there's still magic to worry about - they'd get wrecked by spells. And what's the end result? Judging from the damage I've seen on onehanders so far, the end result is that dual wielding is now as good (raw damage-wise) as using a two-hander, with the exception that you'll probably get more stats from dual wielding.
Critical Hit Bonus: 5% crit is a 1.25% or so increase in damage. It's not an exciting trait, but it uses attributes you might get anyway with a warrior/thief. Kind of a small bonus for a medium commitment - you might still lock yourself out of a 50-20 trait by getting it.
Defender: Increases mah BASE defense, no. The white number in the character sheet. Let me go through those for all 8 characters: 24, 28, 26, 24, 26, 24, 26, 29. Average of 26, which means this trait would give me a whooping FOUR defense. And unlike the next two traits, I won't build defense by trying to get the trait either. Just compare it to simply getting skill. You'd be totally fooling yourself if you deliberately went for this trait to be tanky. At best I could see someone getting it because they wanted 50 agility and 20 constitution anyway.
Attacker and Archwizard: These are a bit unexciting too. I'm not exactly sure how base attack and magic are established, because they are evidently not only derived from strength/arcane. I'm guessing if I went for these I would get perhaps +10 to the stat at best. However, if I really wanted a RAWR PAINBRINGER character, I certainly would build strength or arcane on it, so these are probably something I might end up getting without changing much from how I would build the character if there were no traits. I mean after all it's: This trait requires what you're spamming up anyway and pretty much boosts the value of those attribute points.
Doesn't feel affordable for nightmare though to make such a glass cannon commitment, receiving little cannon and much glass for it.
Elementalist: Said it before, this is cute from a lore/RP perspective but anyone willing to commit 35 points to arcane is highly likely to be a mage anyway, sporting gear riddled with +magic and not really afraid of enemy spell attacks. And the way I play, 35 will is too much for any character at all.
Cold Blood: Getting 35 con/agi on a mage is less controversial than getting 35 arc/wil on a warrior/thief. This trait isn't so bad, a good bonus for any character, without requiring a crazy commitment. I think if you were absolutely determined to, you could get this + Toughness on some characters. Or Defender lol.
A Ruined Pelt for your thoughts, people.