this plugin offers a nice deal of customization, good job!
However the widget_title filter is not used for the widget title. I had to add it by replacing
apply_filters('widget_title',$instance['title']) in the widget function. Could you please include this fix in next release?
Also, there seems to be no caching in the widget (needed for sites with millions of comments), so might need to code our own caching for the whole sidebar.