It would be nice if the tag bar would display the tags when several records are selected. Some color coding or symbol could indicate whether all or some of the records have the given tag. And if it were possible to edit them at the same time, it’d be great.
This is not at all a small request. However, I agree that something like this could be nice, if all the interface details were worked out so that it was easy to understand how it worked, and if it were possible to modify Mac OS X’s token field to support this kind of interface.
I’ve just tagged two 2000+ mailboxes, and it worked great.
Maybe a tiny suggestion: could you select “append” instead of “replace” by default? The former seems less destructive in case of a mistake (e.g. not seeing that the “replace” radio button is selected).