One of the things that I notice that a lot of people (even veterans and elites) forget often is one of the most basic components of animation posing and drawing, which is the flow of the pose. There is much more expression and a more consistent illusion of movement when the pose of the character is stylized to form a "Line of Action"- college taught me that word, although I had been subconsciously adhering to the principle my entire animation career.
Here is a basic example of 3 poses. the green figure is what the figure "should" look like given the animation standard of line of action, red is the pose without influence of a line of action.

It probably doesn't make much sense to you at this point, one pose just simply looks more bent than another one, but the balance of the figures weight distribution on his center axis, and the general direction/flow of movement are being taken into account. To give more examples, I will use animations from around the forum with poses I would consider "stiff" and explain the error and how it could be corrected.

Above is an example of how not to do poses. These poses, although in an intense action sequence, still look stiff and static due to the non-unified direction of the flow of the body. limbs are shooting every which way with no relation to the direction of the hips and ribcage, which is where the center of mass is.
