That's a tough one. Timing and posing are top contenders, but if I had to choose one, it would be timing. Great poses with bad timing can ruin your animation, but bad poses with great timing can still get the idea across. Both scenarios are obviously not ideal, but I prefer timing over poses.
To me, timing is really important because it determines the intention of your actions. You can have a simple head turn, going from one pose to another, but the feeling will change depending on how you transition from pose to pose.
If the timing is slow, the character may appear tired or very cautious, and if the timing is fast, the character may appear nervous or panicky. Or, let’s say you have a character (a student, for instance) picking up a piece of paper (a test). Your poses and body language will tell us how he feels to some extent, but the timing of how he picks up the piece of paper will tell us if the student is nervous, confident, or stressed.
Or, let's say a character is jumping and falling. Your poses could be fantastic, but if the timing is off, then the physics are off. If the weight is off your animation won't be believable or entertaining. If your poses are bad but the timing is spot on, your animation might not be pretty to look at, but the intention and overall feel and believability of your animation will be intact and the story idea will be somewhat preserved.
Of course, if your poses are absolutely horrendous and your character looks happy and proud instead of sad, then your audience will be confused no matter what. :)