There are 15 Traits to choose from in Legend of Grimrock, 2 of them race-specific. Below, you'll find details on all of them, as well as a rating out of 5 on which ones are most and least useful.
Aggressive: You are full of rage.
Effect: Attack Power +4
Recommendation: 4/5 - More damage is always nice, and +4 Attack Power is equivalent to +8 Strength. Front-line fighters will be well-served by this trait.
Agile: Your reflexes are exceptional.
Effect: Dexterity +2
Recommendation: 3/5 - Dexterity is useful, as it adds to Evasion, Accuracy, and your Fire and Shock resistances. The +2 isn't a huge bonus, but you get more for that +2 than you do some of the other attributes.
Athletic: Your body is muscular.
Effect: Strength +2
Recommendation: 2/5 - Strength is nice for adding carrying capacity to your designated pack mule, but the +2 bonus isn't anything to write home about. Additionally, since you only get +1 Attack Power per +2 Strength, this Trait isn't quite as useful as it sounds.
Aura: You have a potent aura.
Effect: Energy +15
Recommendation: 2/5 - A decent Trait for Mages, as +15 Energy can mean the difference between casting a spell to win a fight, or losing. However, it's overshadowed by Strong Mind, which provides more Energy over the long term.
Cold-blooded: You are naturally resistant to cold.
Effect: Resist Cold +25
Recommendation: 3/5 - Cold damage is relatively rare until late in the game, but the cold attacks you do come across tend to be quite severe. There are few ways to resist Cold damage, so this is a decent choice provided you don't mind waiting for it to be useful.
Daemon Ancestor: Your Great Grandfather had fiery eyes.
Effect: Resist Fire +25
Recommendation: 4/5 - Fire damage is common from the mid-game onward. A stray Fireball can often mean death, so picking up this trait is smart for the defensive player.
Evasive: You know better to stay away from harm.
Effect: Evasion +7
Recommendation: 4/5 - A +7 bonus to Evasion is quite substantial, and is on par with some of the better defensive items in the game. A good trait for Rogues or high-Dexterity Fighters.
Fist Fighter: You know how to survive tavern brawls.
Effect: Attack Power +6 (Unarmed Only)
Recommendation: 5/5 - Only pick this up if you plan on using an unarmed character. Weapons are plentiful past the first level of the game, so sporadic use isn't worth this Trait at all. If you do go all-in with Unarmed Combat, though, this Trait provides a pretty huge damage boost.
Head Hunter: You are something of a trophy collector.
Preqrequisite: Minotaur Race Only
Effect: Attack Power +3 (Per Skull in Inventory)
Recommendation: 5/5 - At least one Minotaur in your party should pick Head Hunter. There are only about 5 Skulls in the game, however, so while the damage bonus is substantial, it will have to be spread out if you have more than one Minotaur.
Healthy: You are exceptionally healthy.
Effect: Vitality +2
Recommendation: 4/5 - There's nothing wrong with a little more Health. Unlike Tough, the Vitality boost will affect subsequent level-ups. Moreover, Vitality adds to Resist Cold and Resist Poison, and reduces the duration of Disease and Poison statuses, making this by far the better Trait. Put this on your beefiest character to give him or her a boost that will last to the end of the game.
Natural Armor: Your skin is very thick and armor-like.
Preqrequisite: Insectoid Race Only
Effect: Protection +5
Recommendation: 3/5 - This is a useful Trait, as Insectoids are quite squishy. However, considering your Insectoids will probably be Mages, it's better they avoid attacks instead of taking them, in which case, Evasive is a better Trait to choose.
Poison Resistant: You are naturally resistant to poisons.
Effect: Resist Poison +25
Recommendation: 4/5 - Poison damage is common throughout the game, and between the early and mid-game can be a party killer. Loses its effectiveness later on, but still a good choice.
Skilled: You are learned.
Effect: Skill Points +3
Recommendation: 5/5 - Free Skill Points... how can you say no? You can get more out of these than many Traits, and the number of Skill Points you have is more or less limited by the number of enemies in the game, so this effectively increases that limit. Even if 3 isn't a full level's worth, it can help make up for the Minotaur's starting Skill Point deficiency, or get you to a useful milestone faster.
Strong Mind: Your mind is sharp as a needle.
Effect: Willpower +2
Recommendation: 4/5 - A modest bonus, and the +2 Willpower will go a longer way over the course of the whole game than Aura's one-time benefit. Essential for Mages.
Tough: You are exceptionally healthy.
Effect: Health +15
Recommendation: 2/5 - More Health is fantastic to have for just about any character, especially Fighters. However, you will be better served by picking Healthy instead, as +2 Vitality will add up over time.