The Witch Doctor is a status and summon user... Unlike the Necromancer he uses great wide range and can fight Army style, Hit Run, or Wide Devastation. Witch Doctor also has the second highest regain over time for Mana
The Wizard is Long Range magic and mysticism... Unlike the Sorceress he uses less elemental and focuses on pure magic and can fight, Long Range, Wide Range, and Elementally. The Wizard has the fastest over time regain for Arcane Power (Wizard Mana)
The Barbarian is of Course a vicious tank with moderate speed only lacking in range... He's alot like the old D2 Barb can play, High damage, High Armor, or Rage Mode. The barbarian in this one has to either use skills or dealing damage or taking damage to gain fury (Barb Mana)
The Monk is Close to Mid Range and has very little in common with the D2 Paladin... He has mantras (Similar to Auras) and his mana comes from attacking not over time and taking damage but has many skills that give Spiritual Energy (Monk Mana)
Demon Hunter is Long Range all the way using shadows, and rage attacks... what's special about the Demon Hunter is She has 2 Mana Sources Hatred which can only be gained through hatred gain skills and Discipline which gains slowly over time... Hatred use is your high critical hit and critical damage attacks that use alot of power and discipline is your tactical and evasion skills.
Their All Freaking Awesome.