Envenom works much differently than other abilities that the rogue might be used to, and therefore must be used with certain considerations in order to benefit fully from its capabilities.
Unlike most other rogue abilities, Envenom is classified as a Nature Spell. Therefore, it will have diminished, or even no effect against mobs with high nature resistance or immunity.
It is possible for Envenom to critically hit its target, causing twice the normal damage. Its chance to crit is based on your melee crit chance, unlike other poison effects, which are based on your spell crit chance.
The rogue must also build combo points fast to use Envenom effectively. Abilities such as [Cheap Shot] and [Mutilate] are effective Combo Point builders.