Email replies and AJAX is the highlight of the latest IssueTrackerProduct release.
If you set up your POP3 for email replies you can simply hit reply on the notifications that goes out and continue an issue discussing without leaving the comfort of Outlook/Thunderbird/Gmail/Hotmail.
Another underestimated important feature is the automatically refreshing issue where the content is periodically refreshed (if the content changes) whilst you're on the page so that you know you're always looking at the very latest copy before you take any action.
For the people who've dared to use the CVS, they've probably already seen the new keyboard shortcuts that is inspired by Gmail. It's very addictive to not have to scroll up and down to reach the navigation or to be able to quickly jump to another issue without having to click on List Issue.
And for the Gettings Things Done people you might like how you now can get a list of "You next action issues" on the Home page. This is a clever list that attempts to figure out what you need to do next in that order.