In bug 349044 I'd like to be able to specify that cards should be copied by default when dragging and dropping (or at least, override the default drag action in some instances). I think bug 191400 would benefit from this. I don't know if all the various toolkits would support this directly, but it would be really useful to have. As for the API, I think that either nsIDragService.invokeDragSession should have an extra parameter for the default, or we should be able to set a default/override action in nsIDragSession. I'm not sure of the full implications of this, but I think it would mean we'd be able to get drag and drop working better.
Setting default QA contact so that someone may actually see this bug.