90% of the time when baking a lightmap, you need to bake it with a different ambient light setting. So "just hit bake" becomes "set the ambient light setting in render settings, hit bake and then remember to reset the ambient light setting in render settings".

My favourite part is when you finish a bake only to realise you forgot to set the right ambience before hitting bake.

This really needs to be an extra setting in the bake tab of the lightmapping window and it needs to happen yesterday. By default having it set to "render settings" would be groovy, but it needs to have an override.