The essences have representative abilities of the classes from which they were spawned as.
As an interesting note, the essences also have some abilities that the class does not have currently. One such case is the Healing Rain ability for the shaman Severed Essence, that is actually probably similar to how it will be in Cataclysm. Another such ability is the Radiance Aura for the Paladins.
Patches and hotfixesEdit
Patch 3.3.0 (08-Dec-2009): Added