Very broadly speaking:
- Clerics and Wizards aren't much different from Warpriests and Mages.
- Martial classes all use the Basic Attacks, and rely on some class feature (stances for Fighters, Movement Tricks for Thieves, etc) to alter those Basic Attacks, making them similar to At-Will powers.
Most Essentials classes give you a choice at 1st level that determine many of your later abilities. Paladins choose a Virtue, Druids choose a Season, Clerics choose a Domain, Wizards choose a School, Fighters choose a Weapon (longsword/hammer for Knights, Greatsword/Greataxe for Slayers), Warlocks choose a Pact... You lose some customization, exchanging it for ease of creation.