Multiple contexts would be extremely helpful for dealing with time, for example if you regularly need to phone people in other time zones. The typical context of 'phone' is only partially helpful; best would be a combination like 'phone+atlantic', 'phone+gmt'.

A subcontext (e.g. 'phone:atlantic') is also only a partial solution. You might also need to IM people in other timezones as well, and shouldn't need to enter a duplicate set of time-related subcontexts. I can also think of other time contexts like 'business hours', 'evening', and 'weekend' which I'd like to add as a second context to many of my tasks.

Come to think of it, time context is such an important factor (neglected in GTD?) that it deserves special handling. One's regular timezone could be added as a default time context to every new task, and you could fine-tune as needed (maybe add 'business hours', or 'weekend'). OF could easily make these active/inactive depending on the time of day. When traveling, you could tell OF your current timezone and active time contexts would adjust accordingly. For example, if your regular timezone was Pacific but you're currently in GMT, a task with 'Phone+Pacific+Business Hours' wouldn't be relevant if it's 9am on a Monday, but would be at 11pm on a Friday.

As I think more about this, such functionality would add a level of sophistication to OF that is lacking (as far as I know) in every other GTD app. It would really put it in another category altogether.