Support for folders
http://www.livejournal.com/users/spyderous/50123.html
demonstrates a good reason to have support for organising feeds in folders.
Liferea like folders (or a 'tree' view in the left-hand-side feed list pane) are kind of ugly though, they clutter up the interface and don't look very elegant. You're also asking the user to take on the work of managing the folders.
I think a better approach is to use feed 'categories' like the console feedreader Raggle does. This is how it could work in Blam:
Add en extra field 'Category' in the 'Edit feed' dialogue, category should be a drop down list of already created categories, but the user can also type a category directly into the field to create new categories. It should support one feed belonging to multiple categories. (So what your actually going to need is something like a list of categories with a remove category button and a drop-down list/text entry add category button). Categories are things like 'News', 'Linux', 'Tech', 'Politics', 'Important', whatever the user wants.
Add an extra menu 'Category' to the toolbar, consisting of a single check-list of all the categories that have been created, plus an entry labelled 'All' at the top and an entry labelled 'Uncategorized'. The user can check one of these entries at a time. This list can also be popped up by pressing the 'c' key.
In the left-hand-side pane Blam displays all feeds belonging to the currently selected category. Remember that a feed can belong to more than one category, that all feeds belong to the All category, and that new feeds and others without any category entered belong to the Uncategorized category.
I think this achieves the same result as Liferea folders but is a simpler and more elegant UI design, and it has the advantage of easily giving feeds multiple categories, which in Liferea would involve copying a feed between folders. Once this is implemented, if people want to have hierarchical categories that could be handled automatically in a way similar to how the web browser epiphany handles hierarchical bookmarks with its 'topics' scheme.
I haven't used Liferea recently to see exactly how it works. The categories system works very nicely in Raggle.
Update: Straw implements categories also.
