I've been looking at several GTD suites, and so far, yours has the best combination of looks and functionality,
however,
there are some things that i think can be improved:

Reminders

i know i tend to forget things, and i imagine many people share this problem, enabling reminders by default for tasks (maybe even only for specific contexts, goals or projects would greatly increase this apps usability for me
notification persistence - Astrid for android has a setting which forces the user to look at the reminder before it can be dismissed, i can not begin to count the ammount of times this has motivated to act on an item.

Web Client

Calendar view - this is an invaluable feature, and is what triggered me to search for an alternative for my current GTD suite, i think adding an option for weekly and daily displays, would allow easier time management analysis for users.
Tagging - adding tags from the main views, or from the task view instead of ONLY through the edit screen would make task subdivision easier, for instance: Students could subdivide their courses using labels while having a context for School

Android

Calendar - if you could make the widget weekly instead of (or in addition to) monthly, it would solve the screen space issues (at least on landscape view, and for some devices such as the Note II, in portrait mode as well) and allow users to better manage their time, could also be implemented in app
Widget - Adding a label specific widget would be nice
Widget - Could be made more customizable, i for one would like to be able to have the time\date of an item displayed alongside the item, also, a tickbox for completing could replace the current view/complete buttons which hide the tasks text..
Reminders - What i said earlier applies especially to android

Windows client:

Calendar - Would have been nice to have a calendar display here as well
Usability - It seems to run pretty slow on my computer, it's a midrange laptop, and the app does not seem to be that heavy...

General

Doit Now & Next - I'd have expected tasks marked as doit now, to show up in the next tab...
Filters - Still unclear to me what those are and how i create them
Sync - i've experienced some issues with tasks not syncing properly between devices, sometimes they don't sync at all, sometimes they sync wrong (no label, wrong time etc')
Prioritization - Would be easier to control in bulk if there was a way to assign priorities from a list instead of having to edit the task...
Tagging- Same as above


Anyways,
Great work! i'll be considering a switch though right now the lack of proper reminders is a major downpoint for me