Ticket Validator plugin (https://trac-hacks.org/wiki/TicketValidatorPlugin) is very useful.
It forces users to set a value to specific fields when changing ticket status.
For example, you can make "target date" field to be mandatory when making a new ticket, or you can make "Implemented in version" field to be mandatory when closing a ticket.
We are having an issue that users always forget to fill which revision the bug was fixed when closing ticket. With the plugin, we can prevent the issue by making the field mandatory when closing the ticket.