First off, I love the interface. It's simple, clean, and gets straight to the point. That said:
* What's the difference between inbox and a new list? My first guess would be inbox holds all tasks assigned to me.
* Clicking the "attach label" button and then clicking "close" afterwards leaves an @ character in the description
* After clicking "Add an item", please display some ghost text like you do for status updates so I don't have to figure out what I'm typing
* In the "View all" list view, I suggest displaying only 5 or so tasks from each list
* Tasks should be sorted by due date
* Your link reads "Add an item". This should read "Add a Task" for consistency
* Changing "Completed items" to "View completed items" would be a better call to action
* Make the "no due date" an "Set due date" button instead. The fact that it's a text box makes me click on the "attach label" button in attempt to add a due date
* The label adding popup is effective only if you want to add a single label. Adding multiple labels becomes a nuisance, and the reordering of labels is confusing. You can also add a single label multiple times. Keep track of which ones are already on the task description, and have the user click a label to switch on/off.
If you're going to reorder based on usage, I suggest doing that after the user navigates away from the Tasks page entirely; keep the workflow consistent. Also, let the user click outside the popup to get the same effect as clicking the "Close" link. That really bugged me.
* I'm not sure what I did, but due dates are being saved to one day before (e.g. selecting June 24 and saving results in June 23)
The badges seem really arbitrary; they don't have a sense of accomplishment to them. If you really want to get into game mechanics, let the project manager set the rules and rewards for badges. Allow rules based on not only quantity of tasks, but types of tasks also (e.g. completed 5 @urgent tasks).
I can see this being really useful if you make your real time chat feature a disguise for a web front to irc. The project owner could set the credentials, and boom, you have real time chat with instant integration to many small company's irc setups. You could also host an irc server yourself, and offer to host project chat for a small fee.
Overall I really like the foundation you've built. Its simplicity will easily let you expand your feature set. I think chat and online status will need to be implemented before this can really take off. Adding an etherpad feature would be killer imo. Etherpad is open source, after all.