wee! another former Blender user! That's what I started out with back in the day...
well, back to the topic: the reason for the clear plastic not to look right is that you didn't render with refractions turned on or had your index of refraction set to 1.
To get realistic refractive surfaces, set the refraction to something other than 1 (1.5 to 1.66 for glass, 1.55 for Polystyrene), also you might want to make a copy of your glass/clear plastic surfaces, paste them into a new layer and flip them (hit "f"), then assign an "air" material to it: diffuse 0, spec 0 refraction 1. This way the lightrays get "bent back" when they exit the material.
Come render time, go to the render options and check raytrace refractions, set your ray recursion limit to something like 5 (the 2 air surfaces, 2 glass surfaces and one more) to speed up rendering.
Post a new version here
