1. Give the players of Fighting-Men interesting choices as they level.
2. Get rid of Thieves and Fighter sub-classes while throwing a bone to players that love them.
A sub-goal of #1 is to motivate exploration and the formation of social ties which may offer expanded access to disciplines for the players of Warriors, just as they can grant access to expanded spell lists for Wizards. #2 fairly flexibly satisfies the need to make different flavors of non-supernatural characters: it's easy to mix the tropes of fighter, ranger and rogue to whatever degree is appropriate to the character concept.
In any case, here's the (newly revised) entry from the Characters chapter:
Disciplines: Warriors , even at first level, are veterans of combat with substantial training or natural ability, and thus at least some skill in a particular martial discipline. The Warrior selects one such discipline at first level and an additional one at every level divisible by three. Disciplines are arranged in broad categories to provide guidance in creating a character with a particular focus or specialization. Certain other advanced or secret disciplines may be available in the campaign, but they are the exclusive domain of particular military orders, secret societies, etc. If you are not sure which discipline you should choose, Champion is suggested.
- Pentathlete: gain +1 to checks involving wrestling, running, jumping or throwing
- Marathoner: can maintain a forced march for a full day with no penalty
- Freerunner: gain +1 to checks involving tumbling, jumping, and traversing
- Quick Shot: attack twice per round with missile weapons
- Longbowman: can effectively use the war longbow, dealing 1d8 damage
- Skirmisher: can move and make a missile attack in the same round
- Champion: attack twice per round with melee weapons
- Slayer: make one attack hitting all targets within reach of 1 HD or less
- Soldier: can move and make an attack in the same round without charging
- Burglar: gain +1 to checks related to picking pockets, and disabling locks or traps
- Assassin: may quietly disable with a garrote, sap, shiv, or sucker punch on surprise
- Smuggler: gain +1 to checks related to hiding, appraising, and fencing goods
- Lancer: use your mount’s Might when making a mounted charge
- Cavalry: can move and attack in the same round while mounted
- Horse Archer: can move and fire a bow while riding a galloping mount
- Scout: gain +1 to checks related to surprise and perception
- Hunter: gain +1 to checks related to tracking and hunting
- Mountaineer: gain +1 to checks related to climbing and may climb sheer surfaces