Sap is an effective crowd control technique. Don't forget to tell the party which mob you are sapping, as any damage breaks the effect.
Sap does not cause threat, meaning that if you Sap a mob and then move out of normal aggro range, the mob will not attack you after the Sap effect is broken.
You may choose to Sap a different target. The first Sap target will wake when the second one is Sapped.
Mobs that are Sapped won't be tagged as your kill until you deal damage to them. Be wary of other players stealing your kill.
Sap can be useful in a PvP environment to even out a 2-on-1, to take out a Shaman's totems before you fight him, to capture a flag quickly in battlegrounds, or even after a [Vanish]. Remember, like all crowd-control effects, Sap is limited to a 10-second duration in PvP.
Sapping a player of the opposite faction will turn on your PvP Flag. Be wary.