I think the arm muscle tension is a matter of fine-adjustement, you could have a point there, maybe it would even make the expression stronger if there is more strain...
The nostrils can indeed look like that, it's a very extreme position of the mouth, this. They could however also end up smaller, so I could by all means reduce them somewhat if people think they look unusual enough to detract from the expression.
We'll see about adding more wrinkles to the face, I'm somewhat reluctant to messing any more with it at this point. What exactly do you mean by "plasticky" though, that it's too plastic, or that it's shiny?
Oh, and if I didn't want critique, I wouldn't ask for it. Feedback and suggestions on how to improve is great!