No ability to prevent posting to an issue
Another feature that googlecode had that I'm really missing right now... Redmine has the ability to "private" issues, which essentially hides them from anyone that doesn't have manager or developer status, but there's no way to lock an issue from being edited. Ideally there would be a way to lock an issue while leaving it public, so others can still see it, and that was fairly commonly done on googlecode.
Apparently this is a much requested feature, but that issue report has existed for two years without anyone in redmine acting on it. :/ Various devs have forked redmine and made their own solutions, though.