Well...if we are going to use mechanical differentiation, which I thought we weren't...then I'd see something like this...
Warriors
-Fighter: Strength reliant, Combat Expertise.
-Barbarian: Strength reliant, Combat expertise + Constitution, Power Point mechanic: Rage Powers.
-Warlord: Strength reliant, Combat Expertise + Charisma, Mundane Aura mechanic: Inspiration & Leadership.
-Paladin: Strength reliant, Combat Expertise + Wisdom, Mundane Aura mechanic: Leadership, Channel mechanic: Divinity.
-Ranger: Strength reliant, Combat Expertise + Intelligence & Dexterity, Skills expertise, Channel mechanic: Nature.
Wizards
-Mage/Wizard/MU: Intelligence reliant, Spell use mechanic [various mechanics for spell use and spell schools provide all types of MUs]
-Witch: Intelligence reliant, Spell use + Charisma, Channel mechanic: Nature.
-Thaumaturgist: see Priests.
-Swordmage: Intelligence reliant, Spell use + Strength, Combat Expertise.
-Warlock: Intelligence reliant, Spell use + Constitution, Power Point mechanic: Eldritch Powers.
-Artificer: Intelligence reliant, Spell use + Dexterity, Skills Expertise
Rogues
-Thief/Rogue: Dexterity reliant, Skills expertise.
-Assassin: Dexterity reliant, Skills Expertise + Strength, Combat Expertise.
-Bard: Dexterity reliant, Skills Expertise + Charisma, Mundane Aura mechanic: Inspiration & Fascination, Spell use.
-Monk: Dexterity reliant, Skills Expertise + Wisdom & Constitution, Power Point mechanic: Psychic powers.
Priests
-Cleric: Wisdom reliant, Spell Use, Channel Power: Divinity.
-Druid: Wisdom reliant, Spell Use + Constitution, Channel Power: Nature.
-Shaman: Wisdom reliant, Spell use + Charisma, Mundane Aura mechanic: Fascination, Channel Power: Spirit.
-Thaumaturgist: Wisdom reliant, Spell use + Intelligence, Channel Power: Divinity.
-Paladin: see Warriors.
-Monk: see Rogues.
Additionals
-Psychic: Intelligence, Wisdom & Charisma reliant, Power Point mechanic: Psychic powers.
That's 19 classes, without even trying too hard...ticks me off cuz I feel like there's some obvious #20 combo that I'm forgetting.
And the individual "mechanics" list isn't too overwhelming...
The 3 "Basics", if you like:
1) Combat Expertise: Strength dependent or leaning classes. Self-explanatory.
2) Skills Expertise: Dexterity dependent or leaning classes. Essentially the game presents a Thief's skill set, a Ranger's skill set, an
Assassin's skill set with "Choose-Yer-Own" guidelines for expanding/mix-n-matching skills for additional sub-classes as needed: i.e. if you want "Swashbuckler" as a class, "Acrobat" as a class, "Bounty Hunter" as a class, etc. etc.
3) Spell Use: Arcane, Divine, Natural : mental ability (Int. or Wis.) dependent or leaning classes.
...and the 3 "Add-on's/Secondaries":
4) Channel Power: Divine, Nature, Spirit : mental ability (Int. or Wis.) dependent or leaning classes.
5) Mundane Aura: Inspiration, Leadership, Fascination : Charisma dependent or leaning classes.
6) Power Points: Psychic, Rage, Eldritch : Constitution dependent or leaning classes.