It would be very useful to add a feature, that lets you tag the issues with the defined categories for a project. Today you can assign one category per issue. Technically this would simply mean to add multiple categories to one issue.

The feature would be a lot more fancier, if it was solved over a AJAX-feature. For an example see the tagging for E-Mails in the webbased mail-client at www.zimbra.com.