Into the Dungeon is built on the base system of Into the Odd. As such it has just three Ability Scores.
Strength as physical prowess and fighting ability.
Dexterity as skill with ranged weapons, sneakiness and reflexes.
Willpower as mental prowess and inner strength, particularly when dealing with magic.
So Warrior puts his high score in STR, Rogue in DEX and Mystic and Disciple in WIL, right?
I'm pleased to say it isn't that restrictive.
Warriors get most of their combat prowess and toughness from being a Warrior. Of course, when it comes to dishing out the killing blows and avoiding them in term you're going to want good STR and DEX scores. Which one you go for will depend on if you want me mostly dealing with ranged or melee combat. Both are equally viable for a Warrior. With the open nature of how combat Techniques work I can even see a high WIL Warrior acting somewhat like a 4e Warlord if the player was creative enough.
Rogues are probably the most versatile in terms of Ability Scores. As they dabble in the classes of their allies they can put all their scores to good use. Similarly, their Second Chance ability applies to all types of Save and actually promotes having good scores across the board rather than one super score. When you're playing a Rogue just put your scores wherever you like and you'll be fine.
Mystics benefit from a high WIL score, but might not need one as often as you think. Saves are made against your WIL when you target someone else with a spell and when trying to dispell another Mystic's magic. A low WIL Mystic isn't completely out of luck. Just look at how many spells don't target others. Even the likes of Acid Arrow and Fireball will be useful for wearing down opponents, even if you're not getting the killing blow in as often. If you want a Mystic that can handle a sword or dodge danger quickly a high STR or DEX might not be such a bad idea.
Disciples would seem to point towards WIL again. Sure, most Deities grant an ability that draws on the Disciple's WIL score, but not all of them. Even those that do tend to have at least one Prayer that doesn't use WIL at all. It's also easy to see how Disciples of the Silver Avenger and Masked Trickster would benefit from high STR and DEX respectively.