The todo.txt file format supports extensions, and some of the extensions include start date, due date, recurring tasks. The Simpletask todo.txt Android client supports these, and can filter by things that are past their start date, sort by due date, etc. Todo.txt also supports projects/contexts/lists/tags and Simpletask can filter by them. i don't know which todo.txt commandline clients, if any, can do this but i bet…
One big area where taskwarrior shines compared to solutions like todo.txt or org mode is synchronizing between devices using taskd. Although it is a bit involved to set-up, once it works, it works very well.
I am slowly trying to add task identities to Simpletask to get closer to this ideal (at the cost of 'ugly' meta data in the Todo file)