For me Claws is one of those examples which describe the problem of the Maemo platform and the N900 in particular. Advertised as a tablet it is able to operate like a desktop PC however its form factor is more of the phone type.
If you use it as a PC then you need the stylus to eg. navigate through small size drop down menus or use keyboard shortcuts. If you use it like a (i)phone you definitely don´t want that. In this case the app is either clever laid out to provide a quick touchscreen navigation or functionality is reduced and adapted.
This is my problem with Claws for example- the functionaliy I use mobile is a rather limited subset of what Claws provides and the layout (though it can be adapted) is imho not very suitable for such a small screen. Sure I have some whishes which the built-in mail client should provide, encryption and signing is one of them.
However it is sufficient to quickly check mail from several imap accounts and respond/forward those.

Regarding TaskCoach: I used it a lot lately. And I found out that the power of the app can only be applied on larger screen layouts. Take the various views you can have, the category window, timeline, etc. Most people will have their personal task layouts combined from several views on one display which you can not bring to a mobile device without rendering it unusable. Imho.
Therefore I support the approach the iPhone app was going. A different standalone application compatible with the XML file format which focuses on the primary purpose of the app- providing a glance about tasks, editing them, synchronize them. All in a layout which considers usability issues on mobile devices.
Because this is the use case I believe most people will apply mobile: A quick glance what is to do today, tomorrow, next week, what is open, what is completed and adapt the progress, then write it back to the master.
Keep it small, simple but powerful.