The base damage of any instant melee attack that does "weapon damage" is the damage per attack (not damage per second) of the weapon being wielded in the main hand. If your main-hand weapon is listed as "Damage 141-208", for example, your instant attacks will all do a base damage of 141-208 damage points.
As a result, a slow weapon will do more damage with an instant attack than a fast weapon with the same rated DPS.
To this base damage is added the damage bonus for the maneuver, if any, and a damage bonus based on the wielder's Attack Power.
Stormstrike, Mutilate and Whirlwind
The Shaman talent Stormstrike, the Rogue talent Mutilate, and the Warrior ability Whirlwind are exceptions to the main-hand damage rule. These instant attacks are assumed to be made with both wielded weapons at the same time, if the attacker is dual-wielding.
The base damage of these attacks is equal to the damage-per-attack of the main hand weapon and half the damage-per-attack of the off-hand weapon. Note that the rogue talent "Dual Wield Specialization" (rank 4 talent of the Combat Tree) will increase your off-hand weapon damage from 50% to 75%, which does affect the Mutilate skill. This is one of the few cases where having a slow off-hand weapon will result in more overall damage being dealt than having a fast off-hand weapon will.
Attack Power damage bonus
Instead of multiplying by the speed of the weapon, the player's normal Attack Power bonus is multiplied by a fixed number pulled from the following list:
- Two-handed weapons - 3.3
- Daggers - 1.7
- Druid's cat paw - 1.0 (Uncertain)
- Druid's bear paw - 2.5 (Uncertain)
- All other one-handed weapons - 2.4
Please note that all normal weapon swings are completely unaffected by this.