T is for Tags.
I apologise to anyone who only uses pen and paper to track their to-do lists, because T's subject is only useful for computerized to-do list applications. However, I think it's such an important and useful part of a computer-based solution that you lo-fi (and if paper works best for you then stick with it) system users will have to bear with me.
I may be biased, given that I've written and use a tag-based online to-do list app, but I believe that the ability to tag your tasks is one of the most important features when it comes to managing your tasks.

