Ok, I am ready for another attempt to improve the preferences dialog :)
My brand new idea: why do we need this dedicated "add filter" input field at all? Why can't we use our inline editor?
User interface mockup (put together in Paint):
To add a new filter you press the Insert key or choose "New filter..." from either Filters or context menu. A new line appears at the end of the "Advertisement filters" list (another possibility: below the filter you have selected) with inline editor already activated - and you can enter your filter there. Or press Esc, then this line will be removed again. Note that the OK button becomes enabled once you start typing something in the editor (this is new in Adblock Plus 0.7.0.2) so that you can press it immediately - both to add the new filter and save the changes.
Note that the inline editor has a dropdown list in this mockup - that's where the suggestions are located, so you still can access them.
What happens if you choose "Adblock" from the context menu? The preferences dialog shows up with inline editor already active in a new line containing the address in question. You can edit this address to make it a filter and press OK then - only one click. Or you press Esc if you decide not to add the filter.
Now what if you doubleclick an item in the sidebar that is already blocked? Here we will have to loose a little functionality - we can only select the filter in effect, we can't start the editor at the same time. But I don't think this creating filters from already blocked items is a particularly vital feature.
It is probably not too obvious how to add new filters. Even worse, you can only cancel adding a filter via keyboard. Right now clicking another item or scrolling while inline editor is active saves your changes - maybe it should cancel instead. One exception needs to be made however, clicking OK still should save the new filter.
You can't scroll the list while composing a new filter. This can be alleviated if we choose to show the editor for the new filter below the currently selected one.
Adding filters with keyboard only becomes more complicated - open Preferences, press Insert, enter filter, press Enter, Tab, Enter again. As opposed to: open Preferences, enter filter, press Enter and Enter again.