The straps look fine for thickness to me, though the loop at the bottom does look a bit angular (fairly minor).  If there is any thickness on the helmet it is hard to tell with the shadow falling the way it does.  

I'd consider some more advanced shaders for the straps and metal bits.  Maybe a distressed texture/shader option for the helmet.

It looks pretty good, it just looks very 3d ish so might not be of use to someone who renders more realistic scenes, which would seem to be where your market is aiming.