Her left arm is completely messed up, it looks like it's just extremely short. They really should have made it as long as they other one with a bend in it.
Her other arm isn't much better really, it's more noticeable from the side but her arms really look like they need extending.
The issue with her arm is more an issue of style. I'm not saying it could not be improved, because it could. However this is not a simple case of the anatomical measurements being wrong because we are looking at a style that is very intentionally anatomically incorrect. Which is not uncommon in animation at all Allow me to clarify what I am trying to say with this image...
http://amphomepage.com/temp/lee/Elaine-anatomy.jpg)
Anatomically speaking Elaine's head is massively out of proportion to the rest of her body, that is a design style choice and not a anatomical mistake. Following on from this she also has the very common issue with drawn women, in that her legs are far longer than her upper torso. Check almost any comic book to see this same issue. Now these are style choices that most people actually like, go see a top ten list of popular comic artists and then check the anatomy of their women.
Now this leaves the artist with the issue we are all seeing with the arms. A better artist would be able to hide this, but obviously this person has made a few mistakes and left it more obvious.
The issue I am talking about is this. With such a small torso compared to the arms, with what part of the body does the artists match the arm length? If you match it will the entire height of the person then the arms will stretch far too low on the legs. If her wrists started to approach her knees she would begin to look like a gorilla. What the artists has don is what most would do (although he could probably have gone a little lower) and that is left her arm stretch to the point in the legs that a regular arm would (as you can see form the anatomical model). This prevents her from looking like an ape, but can leave you with stubby looking arms. I think the artist should have gone a little further as I have done below (extending her arms only 7 pixels), but if you go to far you get the issue on the right (extending her arms only 16 pixels).
http://amphomepage.com/temp/lee/Elaine-anatomy2.jpg)
Also totally agree with SyntheticGerbil, the reason the animations look so stiff is definitely because they have been drawn stiff. Bill Plympton is a great example of fantastic animation on a low frame rate.
Don't think anyone disagrees that the drawn frames are a little stiff, and that causes the slight stiffness in the animations. I have said so myself many times above.
However they are also pretty choppy, an issue with the increased resolution and that was what my point was regarding, at this fidelity the animations would really benefit from more frames than the originals and would look choppy not matter how well the frames were drawn. As we have clarified elsewhere these are different issue's all together.