Update 2009-02-05: Now that I've been doing this for a few years, I've refined this tip and its subsequent updates into an article at eHow: How to use Thunderbird to get things done.
Revisiting my previous post, Using Thunderbird to Get Things Done yet again, I recently upgraded to Thunderbird 2.0, which changes the behavior of "labels" into "tags". As a result, I've had to adjust some of the process I described in my original post.
First of all, Thunderbird now lets you apply multiple tags to messages. This was not the case previously. So now I can have a message simultaneously marked "Delete" and "Action Required". This is, as you might guess, undesirable given that the previous process had assumed only one label could appear per message. However, this is pretty easy to work around, but it's a little more prone to human error: just don't apply more than one tag per message. If changing the tag, hit "0' first to remove all previous tags. I'd prefer to have an option that tells Thunderbird "only one tag per message", but that seems unlikely as it violates the concept of "tags".
Secondly, most of the "saved searches" I had set up stopped working. I had to get a little more creative with how they were specified. So now my "Next Actions" search has the following parameters:
Search Online checked
Match all of the following:
* Tags doesn't contain "Delete"
* Tags doesn't contain "Archive"
This leaves "Action Required", "Wait", and "Defer" visible.
Note that the documentation on tags recommends that you use "contains" or "doesn't contain" instead of "is" or "is not" in saved searches involving tags, as the multiple tags thing will bite you: For example, a message that is tagged both "Delete" and "Wait" will not show up in a saved searched for "Tag is Delete", but will show up if "Tag contains Delete".