Important Things When Posing a Character
The most important thing to look for in a pose would be a strong line of action, if it is an action pose. If the character is mid-action, such as pushing/pulling something, fighting, running, jumping, etc -- in any of those, there should be a really clear line of action, especially in your key poses. A single line that describes the force moving through the body and the momentum of the movement, ideally a line running through the entire body, and hopefully into an arm and/or leg. The stronger, longer, and more connected this line is, the more dynamic and strong the pose will feel.
If it isn't an action pose, then you should probably be more concerned with making sure the pose clearly describes an attitude or emotion. You should be able to tell, from one static frame, whether the character is sad, happy, angry, shy, embarrassed, nervous, excited, or afraid. In these cases, it is OK to sacrifice a strong line of action in order to strengthen the emotion, at least in my opinion, although it's often possible to craft a very communicative pose while still retaining a nice line of action. However, showing us the internal thought process should be the priority in these types of scenes...
