Batch update of M2M fields behaviour
At the moment, if you batch update m2m fields, all model instances in the selection are set to the same set. It's often desirable to simply add a value to all model instances, rather than remove all previously-selected values. Similarly, it may be desirable to remove a value from all model instances.
I suggest that for m2ms, the interface is something like:
Remove these values [list] Add these values [list]
Remove happens before add, so if you wanted to accomplish the effect that currently happens, you'd 'select all' for remove, then add the ones that you want.