Sap does not have a chance to break early, always lasting the full duration unless the sapped target is hit. It suffers diminishing returns and duration in PvP.
It is no longer considered an incapacitate effect.
Sapping in a neutral town does aggro the guards.
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.
Patch 4.0.6 (8-Feb-2011): Sap now has a PvP duration of 8 seconds.
Patch 2.4.0 (25-Mar-2008): Sap is no longer considered an incapacitate effect; changed to allow more creatures to be crowd controlled in dungeons who were previously immune.
Patch 2.3.0 (13-Nov-2007): Multiple Saps placed by multiple Rogues will now be handled correctly in all cases.