It would be great to have the ability to change fields on more than one ticket at a time. Here's my proposal for you:

Anywhere there's a list of tickets, have a button that behaved similar to the "Prioritize" button now, in that clicking it would reveal a new control per line. In this case, though, the control would be a simple checkbox. However, it would also reveal another panel of options (perhaps in the sidebar, perhaps in a non-modal popup) such as:

• Milestone
• Who's Responsible
• Ticket Status
• Add Tags
• Delete Tickets (button)
• Comment (textbox)

The way that iTunes handles changing information on multiple tracks at once is a good model. It makes it clear which of the available fields are actually going to be changed on all the selected items.

This would be particularly useful for moving tickets from one person or milestone to another (moreso than the other options, i think).