Well, there's the Zealot trait (+3 initiative, -1 HP per level) available to Wraiths if you're willing to train troops out of an Infirmary Fortress (grants Endurance I trait for +1 HP per level to units trained here). As for why I say 'out of an Infirmary Fortress' - Zealot + Wraith Blood means you have 0 HP per level. Adding Endurance I from the Infirmary to that brings you back to +1 HP per level. Zealot + Fast gives you +5 initiative, which is a little better than Fast + Impulsive (+4 initiative), but means that you've sacrificed whichever trait you would have put into the slot taken up by Zealot. Overall, I'd say that Zealot is bad to mediocre, depending on where you're training units.
Wraiths also have the unit trait Hallowed Rite, which generates 3 mana each time a unit with that trait kills something. This is another marginal trait but can be useful to fuel early spellcasting.
The Race of Men gets the 'Potential' trait for +25% experience, which stacks both with the 10% bonus from the blood trait and the various army experience bonuses, which can let you push your trained units up to high levels quickly to stack the level bonuses, or help counteract the penalty to the army for stacking champions (though this won't do enough to make that particularly worthwhile even so). It's a reasonable to good choice for Henchmen, but probably mediocre at best for regular troops (unless you're really stacking the level bonus traits).
Mancers get the Experienced trait, which is just like the Veteran trait except it also increases unit upkeep costs. At best, this is a mediocre trait, as one extra level does less for you than most of the other traits do, and the additional upkeep costs don't make it much more appealing.
Underdog and Bully are similar to Finesse except that they work on relative level. Having Underdog and Bully on the same unit guarantees that it gets a +3 attack bonus against anything that isn't of equal level. I don't know if these stack with Finesse, but if they do then Bully + Finesse + Fast will get you +6 attack against most computer faction troops, and Underdog + Finesse + Fast will get you +6 attack against many high-end monsters.
Unit Design Traits:
Acrobat, +1 dodge per level, 6 production Balance, +10 dodge, -1 initiative, 6 prodcution Bloodthirsty, +25% attack versus damaged enemies, 20 production Brute, +3 attack versus lower level enemies, 10 production Charge, +1 attack and +2 movement on the first turn of combat, 20 production (can be obtained for free from Watchtower Fortresses) Constitution, +3 health per figure in unit, 10 production Defender, +10 defense when defending, 4 production Discipline, +1 accuracy per level, +1 spell resistance per level, 8 production Enmity, +25% attack versus opposite alignment (Men/Mancers/Amarians/Tarth/Ironeers are Kingdom, Quendar/Trog/Wraith/Dead/Urxen/Krax/Slave are Empire), 12 production Fast, +2 initiative, 15 production Finesse, +3 attack versus lower initiative enemies, 6 production Fury, +1 attack, -1 health per figure, 6 production Ironskin, +3 defense, 14 production Lithe, +10 dodge, -1 health per figure, 6 production Muscle, +1 attack, -1 initiative, 12 production Precision, +10 accuracy, 8 production Rage, +25% attack when under 25% health, 10 production Reap, +1 health each time the unit damages an enemy, 10 production Scout, no movement penalties, +1 sight range, 4 production Spell Resistance, +20 spell resistance, 6 production Stealth, monsters less likely to attack, 8 production Underdog, +3 attack versus higher level enemies, 10 production Veteran, +1 level when trained, 18 production Chain Armor Proficiency, 5 production Plate Armor Proficiency, 10 production Zealot (Wraiths only), +3 initiative, -1 health per level per figure in unit, 8 production Hallowed Rite (Wraiths only), +3 mana per kill made by this unit, 8 production Experienced (Mancers only), +1 level when trained, +1 gildar per turn upkeep (unsure if this is per figure or not), 25 production Road Building (Mancers only), 10 production Potential (Race of Men only), +25% experience gain, 8 production Endurance I (Infirmary Fortress), +1 health per level per figure, free Impulsive (Strike Garrison Fortress), +2 initiative, free Charge (Watchtower Fortress), same as the other Charge
Out of the above, Balance and Lithe do more or less the same thing but Balance has the more severe penalty (-1 initiative from Balance, as compared to Lithe's maximum of -6 health, which basically sets its health half a level behind an equal level equal size unit; Ironeer blood or the Endurance I trait make this a third of a level, while Wraith blood makes this a full level, but regardless this is practically nothing). Fury and Muscle also do more or less the same thing, with Fury being clearly superior as it's once again up to -6 health compared to -1 initiative as well as being half the cost of Muscle. Reap and Rage are kind of useless (Reap's healing is not per figure; if you have a 1 figure unit with Reap, it will heal exactly the same amount as a 6 figure unit with reap will each time it does damage; Wraith Blood has the same issue), and Enmity is a better form of Bloodthirsty if you can guarantee that you'll only be fighting opposite-alignment foes with the unit.
Dodge-based units should probably take Balance + Lithe + Acrobat, though if you want to sacrifice any of these traits to take something else (like Fast), I'd sacrifice Balance first. More or less the only reason to take Precision or Discipline is that you're fighting a good dodge-based unit. Discipline and Spell Resistance can toughen a unit against spells, but a mid-level Mage champion, or even a high level non-Mage champion, will blow through any amount of spell resistance a trained unit can come up with so this is mostly a defense against spellcasting monsters and low-level champions. In my opinion, if you're going to take dodge traits, you should probably go all-out; lots of dodge is much more useful than just a little, and Acrobat is necessary to prevent your dodge-based defenses from being eroded by the accuracy bonus from unit levels (+1 per level, +1 more per level for Mancer Blood, +1 more per level with Discipline; Acrobat will cancel out one of these bonuses by granting +1 dodge per level). Especially with Wraith Blood, it may be tempting to sacrifice one of the +10 dodge traits, but remember that hit chances are computed as (Attacker Accuracy) - (Defender Dodge), and with Wraith Blood + both 10 dodge traits + the 10 dodge robe you can get to 50 dodge (meaning a 20% hit chance for a 70-accuracy unit, about normal for low-level trained troops); dropping down to 40 dodge from this point means that that same 70-accuracy unit hits roughly 50% more often and on average hits for more damage. The difference in the potency of dodge-based defenses gets lower when comparing smaller dodge values, e.g. a 10-dodge unit will be hit 20% more often by a 70-accuracy unit than a 20-dodge unit will, though this is still a significant difference if you're basing your defense around dodging attacks (note also that the higher dodge units will also be hit for on average less damage than the lower dodge units because each figure in a unit makes a separate attack role when determining hit damage). Also note that since units gain accuracy with levels, Acrobat is almost essential for preserving the utility of your dodge-based units against high-level opponents.
Veteran and Experienced are not, in my opinion, worth taking; the production cost is too high and you really don't gain that much from just one extra level, especially if you're training out of a Fortress that already grants a couple extra levels. Henchmen and Sions are an exception to this rule, as they gain traits like champions do and so a free level may be a good deal for them.
Scout is a useful mobility enhancement for your armies, but you'll have to sacrifice something for it. Might be worthwhile if there is a lot of rough terrain around your territory and your champions have compasses or some other way to make up for terrain penalties (e.g. mounts when you're using infantry troops). Stealth isn't worth using except on lone units.
Brute, Underdog, and Finesse all offer about the same thing with different conditions; of them, Finesse is probably the most useful as it's relatively easy for the player to ensure the initiative advantage over computer faction troops and most monsters. None of these are bad, though Underdog will lose its utility if you keep your troops alive and develop them to a high level.
Constitution and Ironskin are both good traits for blood tanks. Constitution grants 1.5 levels worth of health to an average unit, which isn't great but helps with tanking or to counteract health penalties (e.g. Wraith Blood, in which case Constitution grants 3 levels worth of health, or Lithe or Fury). Ironskin makes a unit generally harder to kill and can be especially valuable in the early stages where at best you have leather armor. Defender is good for a tank that is meant to sit still and block (possibly with counterattacks if using a sword unit; this is a case where the initiative penalties of Muscle and Balance might be less of an issue than the health penalties of Lithe and Fury, as the unit is just intended to block the enemy's melee line rather than to be a main damage dealer, and initiative is not that important in this role), but is mostly useless on a unit meant to make attacks on its turns.
^ from joeball123
Henchmen & Scions
Henchmen are very useful, as they're essentially design-your-own heroes. You can give them more or less any of the traits that regular units can be trained with, they can use items like standard champions can, they gain the bonuses granted by the city they're trained in, and you can give them a much better version of Potential at training time. They can learn real magic if you have access to New Pariden's spellbooks (either a custom Race of Men faction with the Decalon trait, or access to a a faction with that trait that researched the required techs for the spellbooks to appear in the shops), and have a decent set of minor magics. Train them in a decent fortress and you have an essentially limitless supply of level 5-ish Commanders to put in your towns or buff your armies, and unlike stacking normal champions with one another, stacking henchmen cause no additional reduction in experience gains beyond the effect that army strength has. They're also in an easy spot of the tech tree to reach, they're immortal, and equipping them with the best weapons and shield your faction has to offer doesn't cost gold. (Also, the training them in fortresses thing isn't just with Commanders; it'll work with all classes. Commanders are just the ones which give you the most benefit for doing it; Warriors, Assassins, and Defenders are kind of niche, especially if you already have decent troops, and generally speaking true Champions are better Mages early on, since Mage Champions typically come with a spellbook or two, which means less gold blown in the shop, although the Magic Adept paths that Henchmen can have provide them with adequate access to spells, and there's an advantage to being able to choose every trait that a Mage gets. Henchmen additionally have some decent army-boosting traits that nothing else gets.)
Sions, on the other hand ... I can't say that I'm terribly fond of Sions. They're much higher up in the tech tree than Henchmen are, they are (or at least were, the last time I bothered with them) more expensive than Henchmen are, they have no real access to magic, and they're much closer to being regular units than to being champions (for starters, they die and you cannot equip them with the stuff you find in loot). On the other hand, the description of "best Fallen warriors" is theoretically true - on a per-figure basis, if you train them out of a Fortress with the unit buff enchantments, they'll have the highest base attack and defense skill of any of your units, after they get a few levels on them, provided that your champions haven't been reading too many attack and defense boosting books, but the real champions get access to much better weapons, which more than compensates, and the real troops are around in greater numbers and are easier to mass anyways, which also more than compensates since that both negates the per-figure damage advantage and greatly overshadows the Sion on the HP front.
^ from joeball123