I'd much rather lose the Barbarian than the Druid. The Druid has been it's own distinct class since 1st ed, which it's own approach to spell casting, and integral shape-shifting.
Barbarian? It's a tech level not a class.
But for my own guesses from the scanty data we have so far...
Fighter: The supreme tank, with weapon related special abilities that will tend to counter his opponents actions. (Tripping, sundering, creating weak spots)
Barbarian: The melee damage machine, with special abilities that will tend to make himself tougher, or immune to his opponents abilities. (Poison resist, enchantment resist, flanking immunity)
Cleric: The God of healing, with holy powers. (Turn undead, domain powers, protection spells)
Druid: Master of shapeshifting, strong healer. (Wildshape, Wild Feat-like abilities, nature spells)
Rogue: Backstabbing skill monkey.
Ranger: High damage skirmisher. Mobility is key.
Wizard: Magic damage and controller. (Summons, Fireballs, Terrain shaping spells, Utility spells)
Sorcerer: Bloodline Mage, Heritage related abilities. (Bloodline related magical talents, some warlockery)
Warlord: Party Leader and Master Seargant (Decent melee, Party boosting auras and special abilities (Like the Marshal eats Bard and Dragon Shaman))
Paladin: Holy Warrior (Decent Melee, Divine Feats, Domain Powers)