The best compromise that I've found between plain text and a structured to-do sytem is TaskPaper. It's more or less a to-do enhanced notepad and it's the only to-do system that I've manged to stick with for more than a week. I think it's because it's so free-form that it works for me. Like a dynamic outline.

If you're a mac person, it's worth checking out: http://www.hogbaysoftware.com/products/taskpaper