I'm not a fan of the Mental Attributes in DnD.
It is absurb to assume that they possess even half the weight of the Physical Attributes. Even in 100% political campaigns, there is no way to make these abilities useful (under the present design). Because no mechanic in the game forces an "opposed roll" of Charisma (or any other mental stat), its use in the game is almost superfluous. Since the Mental Attributes have no "direct" effect on the game, short of bonuses to skills that either... never get rolled... or shouldn't be skills at all (more on this later), they are essentially dump stats, unless your class specifically requires their use.
And since attributes should be useful in game play for ALL classes, essentially, these abilities do not hold the same value as the others. They aren't put to good use, they shouldn't cost the same, and they desperately need an overhaul.
Presently, Intelligence provides more skill points (that help few classes other than rogues and rangers), Wisdom grants a bonus to Will saves (and an AC bonus to monks), and Charisma does nothing (although some people argue that sorcerers use this stat... wait... listen for it... that's your arguement getting nailed to a plank of wood).
These stats could be removed from the game, and the game would change less than 1%.
If you don't believe me, try it.
Run an adventure without them and assume your spellcasters have a 15 for their spellcasting attribute.
[Str and Dex are used ALL the time. There's an entire chapter devoted to swinging swords and axes at your enemies! Even Constitution, as a rigid and focused as it is, sees more use in the game.]
The first step to balancing these attributes is to treat them as two entirely different stat pools. Depending on your method, they should be determined separately. If you roll stats, roll Physical ones first and place them among those three as you like. Repeat with the Mental ones. Notice, you have high stats and low stats in each grouping.
The same effect can be accomplished if you use the superior point-based system provided in the DMG.
Next, whenever a race gains a racial bonus to a physical stat, this means it suffers a racial penalty to ANOTHER physical stat. Not the half-assed mental attribute penalties that have been passed off as "balanced" in the past. Go through your race. See if the bonuses match up. Oh look, the dwarf doesn't. And that's on page 1.
This already is a huge fix, but it's only the beginning of making these stats more useful in DnD.
Next, each mental attribute receives a minor to major overhaul in its game function.
In my over-simplification of the stats, Intelligence represents reason and knowledge. It probably means a lot more on some level, but for the purposes of game play, that's all it can measure. However, since the player provides all the reason, problem-solving, and decision-making for his character, the Intelligence score is merely "math" that affects the numbers of the DnD resolution system (which is 99% combat-oriented).
Unless you want the PCs making a die roll for every situation and puzzle they face? Intelligence cannot replace the intellect of the player and therefore, if the PC can't figure it out, a die roll should not be used in its place.
Intelligence is no longer just a bonus to Knowledge skills (which will be changing in a later chapter) you never use or an increase in the number of points you can put into constantly-rolled Craft Skills. Intelligence, now, represents three additional vital additions to the game and one spellcasting bonus. [You will also see new skills in future chapters, but we're not there yet.]
Defense. Intelligent people tend to avoid getting hit with a weapon; even animals avoid getting cut open by predators whenever possible. Someone who is watching the battle, or who has a basic understanding of... sword cuts skin... is more likely to avoid getting hit than a barbarian who throws himself into the fray with abandon. So, it makes no sense that the rules do not somehow represent this.
In addition to a Dexterity bonus to a character's Armor Class, Intelligence as has its own impact -- although not in quite the same manner. Whenever a character selects Total Defense or Fighting Defensively as his combat action for the round, he adds his Intelligence bonus to his AC for that round. Spellcasters, that cast defensively, also increase their AC by their Intelligence bonus, but only by half (round down, minimum 1). Obviously, these are based on Intelligence bonuses, and PCs with an Intelligence below 12 gain no benefit.
Memory. Anytime a character is faced with something he has faced before, needs to recall something from a previous session, or wants to remember the orc word for stop, he is allowed a DC 15 Intelligence check to remember a detail that the player himself may have forgotten or not written down. Don't forget, your characters are living THEIR lives 24/7. Games are an escape from our everyday lives. We can't be expected to remember something that was said six sessions ago.
Tactics. During a battle, Intelligent people (generals) are more likely to find flaws in their enemies defenses. A character that spends a round studying his enemy or aiming a ranged weapon, gains a bonus to his next attack roll equal to his Intelligence modifier. This bonus is good for the first roll made against the opponent in the following round.
Arcane Spellcasting. Not every class can cast spells, but those that can should be able to use Intelligence to their beterment. Any roll made to defeat an enemy's spell resistance that has been "defeated" before, gains a bonus equal to the caster's Intelligence modifier. If I remember correctly, the beholder's left eye was were the acid arrow really made its mark.
Presently, Wisdom is nothing more than a bonus to Listen/Spot and Will saves. That's a weak representation of life-long learning, street smarts, and an overly perceptive mind. While it's true that Wisdom should increase Listen/Spot, it should also represent the "growth" of a character.
Where Intelligence is book smarts and applied logic, Wisdom is an understanding of the intangibles -- people, situations, and the incalcuable human factor. Every epic has the moment where the protagonist learns something; anything. Sometimes the knowledge comes from a mistake or a victory (Beowulf, Wis 11 or 12). But sometimes, the knowledge comes slowly and awaits the protagonist on his death bed (Wis 6) or while waiting in line for Star Wars III (Wis 3).
In addition to the effects on Perception (see later chapter), the character gains an XP bonus equal to 5% for each point of Wisdom bonus and suffers an XP penalty equal to -5% for each point of Wisdom penalty.
Personally, I'm not a big fan of the XP system in DnD. But, 3.x is a great advancement from the Byzantine point structure of prior editions. It's not perfect, but it's all we have. DMs that don't have a problem with PCs not all being the same level, can take full advantage of this idea. And while I don't like percentile XP increases either, this is an excellent use of Wisdom and finally makes it a stat worth putting points into for Fighters and Rogues. Imagine someone playing a wise Barbarian. C'mon. Imagine it.
For about 30 years, players have been making the 18 Charisma joke about being handsome/beautiful, missing the entire point of this attribute. Charisma does not represent looks, but presence, moxie (or lack thereof), and overall command. A high Charisma score can mean anything from good looks to a commanding voice to an aura of confidence. Charisma draws people in or pushes them away.
An ugly person can still have a high Charisma, because he is a powerful or determined leader. I'll bet Grima Wormtongue had a Charisma of 18 or more. Beautiful people usually have a Charisma of 10 once they open their mouths.
In DnD, there is no solid interpersonal system that matches the complexity of the combat system. If a 3rd party company has made one, I haven't seen it. But I intend to show in a later chapter how Charisma will become its own useful tool beyond a bonus to Gather Information checks.
I'm not sure of the full impact of this, but since Constitution only provides a bonus to Fort saves, and hit points, perhaps it could also represent how many hours a day you can work (real work) before requiring 8 hours of rest. Every encounter that involves combat, no matter how long, drains 1 hour from this total. A character with a Constitution score of 12 can march for 12 hours, set up camp, and collapse. A character with a Constitution score of 17 can march for 13 hours, forage for 1 hour, hunt for 1 hour, fight three orcs, and still have time to cook dinner for the party, while setting up his tent, grooming his horse, and setting out his clothes for tomorrow.
Characters with Constitution scores below 10, slow down the party considerably.