Warriors & Mages, Experts & Sages

Combat is a significant part of D&D, and perhaps more so with recent iterations.  It may not be the defining aspect of the game, but physical conflict is a core element and it's presence generally much anticipated.  Further I'd put forward the idea that an adventuring class that has less useful in combat the ability to contribute feels a little lame while the combat monsters are dragging out the big guns.

But at the same time combat is not all there is to the game.  Some games focus also on exploration, investigation, realm building, social interaction, magical research, social interaction and other areas.  These aspects of course could simply be abstracted and left handled without direct rules support, but why not make some attempt to force characters a little bit to expand in other areas?

Adventuring Classes:  Warrior & Mage

All characters gain one general feat (not warrior or mage specific) each level.
There are only two classes which adventurers may progress in:  Warrior or Mage.

Warrior
Warriors are proficient with two categories of weapons training by default.
Warriors gain +1 Attack/Defense bonus per level
Warriors gain 1d10 hit points per level (or 5 reserve points).
Characters starting out as a warrior gain a warrior feat at first level.
Characters may gain one additional warrior feat for every 3 warrior levels they gain.

Sample warrior feats
Some feats may be taken multiple times at the GM's discretion, bonuses from taking the same feat twice typically don't stack.
Cut & Run - As long as one of your actions is used for to attack, you get a free movement action during the round.
Two weapon combat - May fight with two single handed weapons without penalty giving an extra attack per round.
All weapons proficiency - Proficient in all weapon groups.
Favored Enemy - Gain a +4 bonus against one category of creature.  You automatically know the dangers and weaknesses of common and well-known creatures of that type, or can guess several likely dangers and weaknesses of more unusual variants.
Favored Environment - You can use a specific environment to your advantage when fighting gaining a +4 bonus.
Imbued weapon - Some form of arcane ritual, or alliance with a deity, spirit, or other esoteric entity allows you to empower any weapon you use to gain a +1 magical bonus to attack and damage.  There will be some limitation on this however:  It may not work against those your patron views with favor, or may fail against a particular materials or effects (e.g. an ability empowered by a wood spirit fails to hurt plant-creatures).
Dodgy Archer - This feat allows a character to make ranged attacks even if others are engaging them in melee combat.
Fire Into Melee - Your aim is precise enough that you can fire into melee without risk of harming allies.
One With Mount - Any animal you ride becomes as if a part of you.  You and the mount each gain a +4 bonus in combat and to all shared actions.
Sneak attack - Gain 1d6 bonus on attacks that the opponent isn't expecting.  This bonus increases +1d6 every three warrior levels.
Supernatural Strike - Some discipline or quality you've acquired empowers you.  Although your attacks do no additional damage, they are able to harm creatures which are untouchable for supernatural reasons.  This includes intangible creatures as well as those only hurt by weapons of magical nature.
Sweep the floor - You may make a single melee attack against each opponent within reach, provided they are your size or smaller and half your Warrior level or less. 
Tougher - You increase all your hit dice one size.  This includes any levels you've already taken, even levels in the mage class.
Weapon specialization - Gain a +1 bonus to attack, damage, and defense when wielding a particular category of weapon.


Mage

Mages are proficient with only two specific weapons by default.
Mages gain +1 Attack/Defense bonus every 2 full levels
Mages gain 1d4 hit points per level.
Mages able to command esoteric powers, typically called "spells" (see standard spell progression).
Characters starting out as a mage gain one mage style at first level.  When choosing a particular magical style a mage must decide if it is a material style or innate style.  Spells cast with an innate style consume no material components, however the magician can not automatically create magical items related to the style.  Spells cast with a material style mostly require material components, but the caster may automatically know how to create magical items relevant to the style.
Characters may gain one additional mage feat for every 4 mage levels they gain.

Sample mage feats
Some feats may be taken multiple times at the GM's discretion, bonuses from taking the same feat twice typically don't stack.
Armored Spellcasting (Light) - Magical use is difficult when wearing armor.  With this feat any armor classified as light may we worn without penalty while casting spells, but only spells related to a single magical style.
Armored Spellcasting (Medium) - This feat requires and extends the Armored Spellcasting (Light) feat, allowing it to be used for medium weight armor.
Geomancy - Recharge your energy reserves at places of power (typically ruins, temples, etc.). Places have power ratings from 1 to 10 indicating the number of reserve points recovered per hour. The rating is also added as a caster level bonus when casting spells related to the nature of the place.
Magical style - New magical styles may be treated as feats (see list).

Sample mage styles
Magical style:  Adept - Your magical abilities arise from a keen understanding of some narrow aspect of the universe (e.g. fire, the planes, healing or the body, food, etc.).  Intelligence is your prime ability for powers related to this style.  All your abilities must be related to a common theme.  You may record your understandings in the form of written spells accessible to wizards and may make any magical items closely related to your area of understanding.
Magical style:  Attuned - Your magical abilities stem from an intimate connection to some aspect of the universe.  A connection to nature is typical, but connections to a particular element are also common and affinities for machines, emotions, art or other areas are not unheard of either.  Wisdom is your prime ability for powers related to this style.  All your abilities must be related to a common theme.  You may make magical items closely related to your area of attunement.
Magical style:  Imbued - Your magical abilities stem from powers innate to your own physical or metaphysical existence.  Creatures with elemental, fae, draconic, celestial or infernal ancestry are common candidates for imbued status, though others may be imagined.  Imbued powers are for the most part non-material, however magical items may be created if the mage expends a portion of her physical or mystical essence in doing so.  Mechanically this involves spending XP, and magical items created must be in some way related to the mage's imbued abilities.  Constitution or Wisdom may be chosen as your prime ability for powers related to this style.
Magical style:  Priestcraft - You minister on behalf of divine beings and in return are granted specific miraculous powers.  You can prepare any spells of appropriate level from your patron's spell list each day, up to your maximum known.  Wisdom is your prime ability for powers related to this style.  Virtually any sort of magical item may be constructed by a priest, so long as it fits with the deity's theme and is intended to serve it's goals.
Magical style:  Psionics -You either have natural talent or have trained your mind and body to perform extreme paranormal feats.  Psionic abilities are typically called disciplines, rather than spells.  Psionic abilities are almost exclusively innate, and only in a few limited cases can psionic items be created.  Such items typically can allow another psychic to better focus an ability (as with certain crystals) or can be imprinted with psychic impressions.  Unlike spells, most ongoing psionic powers require concentration and are disrupted if the psionist takes physical damage (as opposed to reserve damage).
Magical style:  Shamanism - Like priestcraft, but more flexible, can negotiate with any local spirits.  Unfortunately this also means that if the local spirits aren't conducive then you're out of luck.  Shamans do have a totem always present with them which grants the same spells all the time.

Magical style:  Sorcery - Sorcery involves summoning creatures to your service.  Typically these are infernal beings, but elementals are also common as are nature spirits.  Other beings may also be proposed.  Charisma is your prime ability for powers related to this style.  Material versions of this style involve binding an entity to your service.
Magical style:  Wizardry - You may learn individual spells associated with the disparate adept schools but may not invent spells of your own.  An individual spell is considered innate or material depending on the nature of the inventor, you'll have to take what you can find.

Specialization - Within your style you have a specialization.  You can cast spells within your specialization with enhanced effects by treating them as having a higher cost.
Enhanced 

Non-adventuring Classes

Although warriors and mages are the only adventuring classes, characters still retain the ability to gain skill advancements and feats in other areas.  In fact, since character level is only a measure of combat and magic ability, it is not uncommon for otherwise vulnerable NPCs to be far advanced beyond the PCs in certain types of skills and feats.