Thanks for the response guys

....did you try looking at Castlevania? Those whip animations always looked pretty great...
http://www.castlevaniacrypt.com/games/dx/sprites.htm
Yes, that was one of my first thoughts. Unfortunately the character design has a small but important difference; in Castlevania, the player sprite doesn't hold his whip in his idle pose, so they never have to animate retracting the whip.
I considered changing my design to mimic that, (it would certainly cut down on frames,) but as the weapons are equip-able, it has to be clear at a glance what a given foe is armed with, and what the player will be attacking with.
Try having him turn his body as he whips. Also, his legs look very stiff. You don't have to move them so much, but suggest his legs are at least turning. One trick I always do is to do the animation myself in front of a mirror. The animation is also similar to throwing a baseball, maybe looking at some of those references can help you.
The body below the shoulders hasn't been animated yet, as that will all be secondary motion and will follow the head and shoulders, so I agree that it looks stiff at the moment, but I can't change it until I've finalised the motion of the shoulders, arms, and head.
This is also is probably why it isn't clear that the upper body is rotating (or rather,
will be). As the left shoulder comes up, the texture on the torso in the final animation will be shifting to the left, and the knees and ankles will show the rotation as well.

The thing I'm most concerned about is the motion of the whip itself, and how it behaves after an impact. I'm wondering if there is a solution other than having it pulled back over the head
like this.