Show new comments and changes by marking the issue as unread (font-weight: bold)
I'm using redmine in production environment for a few days now and it's quite impressive! Good work!
What I think is definitely missing is a possibility to show all issues having unread comments or all issues which have changed in one way or another (i.e. new attachments, category changes).
I think one could easily store a "last viewed date" per issue and user and mark all issues as unread which have an updated date which is greated than the last viewed date.