[henry]

Originally Posted by JorBru

I think David Allen would agree that some thought about when a task is to be completed is part of the process.

No, actually I think he would disagree. Nothing I have ever read by David Allen has mentioned this level of formal task planning. The only mention of when a task is to be completed is what he calls "hard landscape", tasks that absolutely must be completed at a certain time. And these are generally imposed by outside forces NOT our own desire to do the action at a particular time.

To the extent that I have captured Context and Estimated Time data for action items, the system could tell me when I have an impossible plan (either because it requires more than 24 hours in a day, or incompatible Contexts, such as being in Boston and Chicago on the same afternoon).

As it stands OF (and GTD) can never create for you an "impossible plan" because the system doesn't impose it's will upon you, it merely offers suggestions on what you might do next based a a few criteria, context, time available, etc. You are asking for the addition of a "Do date" which could lead to plans that are impossible to achieve and is going to need the addition of a conflict resolution system.

add a couple tools for quickly re-assigning "Do" date

And, as you say, conflict resolution will require extra tools because no tool can guess which tasks can be deferred even with the addition of prioritization (please no!). You will ultimately spend more time administering your lists than actually doing the work. Something David Allen points to as a failing in other systems.

I don't see a conflict with GTD here. Rather, I see this as a more complete implementation of Allen's vision.

This is polar opposite to GTD.

Not recognizing those impossible plans means not realizing that Action due dates might not be met. As the work day/week wear on, there are inevitable disruptions to the plan - the sooner that the implications of such disruptions are understood the better. Often, the most important tasks to be completed are important not because their due date is approaching but because their Context is available now and will not be available again in sufficient time for a downstream task to be completed.

So because of a seemingly simple addition like a Today List we have opened ourselves up to additional work of pre-planning our tasks, while trying to avoid impossible plans, and then having to re-plan everything due "inevitable disruptions" ("Inevitable disruptions to the plan" are one of the reasons David Allen argues against Today lists.). None of which is actually doing work. Doesn't sound like stress free productivity to me.

Where OF disappoints is as a planning tool,...

Just as a screw driver will disappoint as a tool to hammer in nails.
Omnifocus isn't a planning tool, it's a personal productivity tool. You should take a look at OmniPlan instead.