The so called 'class mechanics' are slightly different and the classes use different ability names, but the core mechanics are very generic and similar across the board.
A
rough comparison:
Ironbreaker = Black Orc
Swordmaster = Chosen
Engineer = Magus
White Lion = Marauder
Witch Hunter = Witch Elf
Bright Wizard = Sorcerer
Shadow Warrior = Squig Herder
Warrior Priest = Disciple of Khaine
Runepriest = Zealot
Archmage = Shaman
Some careers have variances to make them unique and some don't pair up as well due to 4 classes being cut while in beta.
http://warhammeronline.wikia.com/wiki/Mirror_career