struct vs. class

[Martin]

Inspired by the recent discussion about iterator structs/classes I wanted to ask - what's the design rationale behind having both structs and classes? In C++ the necessity was given by backwards compatibility but that doesn't apply to D of course. 
The only principal differences I can see (I'm not a D programmer (yet), so please correct me if I'm wrong) are a) that structs don't contain run-time meta information (a pointer to a vtable or however that works) and b) that struct variables are statically allocated as opposed to class variables which always seem to be pointers into dynamic memory.
Somehow this looks really unorthogonal to me. Especially b) I find strange, can somebody explain the reason for this?

cheers
Martin