I wanted to create a filter which showed all tickets which were "New" - those which had not yet received some kind of reply. In this way, multiple customer support techs can handle a queue, and take new tickets from the stack.
Working with Adam at FogBugz, I was able to make a filter using -alsoeditedby: - that is "not edited by" and listing the relevant users there. However, this is a bit clunky. It works, and that's fine, but it was difficult to find. I expected to find a flag for just "has replies" or doesn't. So, perhaps in the future?