This list, like the Blur slider below it, must search the selected object's filter stack for feBlend or feComposite, display the current mode of that filter and let me change it (or add a new filter to the top of the stack, but only if no such filter already exists in the selected object's stack).
That would interact very badly with many interesting filters, as they may be built out of multiple feBlend/feGaussianBlur elements; simply picking the first or last feBlend element (or even all of them, depending on how the filter is designed) won't give desirable results.
OK, that was just one of my ideas, I don't insist on making it that way too much. It may indeed be confusing sometimes, although also useful IMHO. What do others think?
I agree that the user should be in control of exactly which instance of a filter he is editing - he might have a blurred object that is then composited which he wants to again blur after the composite. There is no way you can code the dialog to determine what instance of the filter he intends to adjust.
I'm not familiar with the current implementation, so please pardon my ignorance. However, I think it is necessary to have a list of filters on a pane of the dialog (much like the layers dialog) in order of implementation for the object/group which can easily be chosen for editing. They could then be named (?) and kept track of that way if so desired. It's a familiar method already, and seems to me the most intuitive way to control the filters.
Of course, then you might even have some sort of indication (maybe an icon) of what filters are affecting the object, but which are applied to a different subset of objects (group), which could then be chosen and edited. It might be fairly complex, but something like this: choose the filter, a dialog asks if you want to switch the selection to the group affected by the filter (if it's different from the current selection).
Make sense? It does to me, but I'm the one thinking it...