I just happened to compare the field configuration form between D6 and D7, and while I saw many nice improvements, there's one major flaw and regression:
You can specify (upload) a default image file, but that default image file "applies to the Image field everywhere it is used."
huh? This makes no sense. I can perfectly re-use my field_image in various bundles, perhaps even across different entity types. But it's very unlikely that the same default image can be used everywhere.
The default image file information only seems to be stored in {field_config}.data. There is also a {field_config_instance}.data. So moving this info should be easy.
I've skimmed some other issues related to field default values, but those are targeting default values that actually store data (e.g. text fields). In the case of a file or image field, if no file is uploaded, we should also store no data, and only overload the default value if there is no data (IMO).
If we really happen to store a file id reference to the default file, then be it (makes little sense to me though). We'll still do that, but the default image file id should come from the instance, not the field.