Woooooooow, this looks WICKED Dubean! The eyes are perfect!
As far as critiques go, as you said the chin sticks out too much, and I think the jaw may be a LITTLE wide, it's probably just me though, it's hard to tell.
But really, slope the chin down as you said, and then BAM! The most perfect Iron Man helmet!
Heres a couple of pics I put together if you need any further references: