Before I gave you this list of what didn't work for me from the existing applications. Now I would like to give you an idea of what I have found that seems to work well or has promise for the future.
As I mentioned in the previous post, Chandler appears to have the best thought out model of the domain. Anyone looking at designing anything in the domain should take a very close look at their requirements and analysis of the domain.
The apparently defunct Aethera project (it looks like they stopped work on it in 2005) introduced me to sub-tasks which I think are a very good idea. TaskCoach also seems to have looked seriously at the subtask issue.
The discontinued ECCO tool comes close to how I want to handle metadata. My wife introduced me to its ability to link ad hoc metadata to presentation or reports in a way that makes information about a schedule accessible at the drop of a hat. I think that the handling of tagging and categorization will be critical to creating an application worth switching to.
There was a DOS e-mail client called Coordinator that allowed you to track the state of conversations in e-mail. It took some discipline to use but gave back a large amount of benefit.