Alas, my search seems to be over. I came across NoteCase a while ago and have used it for the last couple of weeks. It's a clean and simple 2-pane outliner written in C and Gtk. It's free and open-source software released under BSD license. It does exactly what I want it to do, build a clean and simple outline with no-fuss and no-muss. It also does things like export outlines to html and text file formats. NoteCase will even export to an .exe file which I believe just creates a standalone instance of NoteCase itself, preloaded with the outline you've exported. Nice.
It has all the node editing features you'd likely expect, along with standard text formatting, search and replace, and date/time insertion features. The current version is 1.6.9.
All in all it's a simple, fast and clean outlining program - exactly what I've been looking for. Check your distro's repositories for it, or go here for the different downloads that are available.
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