cooling is the main issue - the inside can get hotter then you think, and airflow over the whole inside matters.
secondly, even if some things get hot locally, be aware that wood burns
3rd - wood warps
4th - plywood or other treated wood emits fumes that might affect the pc, and you, when heated....
5 - the metal/plastic of a typical case transmits its own share of heat that wood probably won't
But if you are a good woodworker and can manage all that, I think a wood case would be very cool to have..