Aha, I was already wondering if the title="flag private message" in your other post should mean that there was something wrong with the PM.
IMHO: chaos, if the functionality of the same icon is depending on the context.
Does this lack of symmetry cause great chaos on my website, or are Users smart enough to figure out what the Red Flag icon means in each situation?
Maybe some users are smart enough (but for them it is not user friendly), but I guess most visitors will not click on the red flag to memorize that it is "important", afraid to report something to the admin.
The star for "important" or "to do" or "to remember" is much better.
All gmail-users are common with it (a yellow star over there).
I should not use a red star, because of the association "red must be something wrong". In traffic lights: red=stop, green=ok, orange="attention". To avoid that a user can think that a green star is "already done", I should go for an orange or yellowish color.
I should say: no issue. - As gmail user I've never missed the star inside a message, nor the option to "star" a message while I'm reading it. When I close the message, I come to my INbox where the stars are listed: you can easily star the message, and see which other ones are starred. There is the overview: the stars are to distinguish the messages. You don't see that inside a message.
when a User opens up a "flagged" Private Message, at this time they wouldn't see anything *inside* the PM to denote this flagging. Is this an issue??
So I think you can sleep without the stars inside.
For me that's N/A, is saving lots of work too. :)
If I decide I want Users to be able to "flag"/"de-flag" a Private message while viewing it, ...
I am not sure where is the best place to place a Flag *inside* the Private Message.
Do I want to add an additional Icon into my website?
Maybe I should leave the Red Flag purely for flagging a Profile or Message, and then maybe use a drastically different icon (e.g. Yellow Star) to denote "Flagging if a PM" ??
I should not use the word "Flag" for description of the stars, while "Flag" is associating with the other "negative" flag (which is also well-known for all visitors of any forum).
Google is using "Without star" and "With star" in the tooltip, and I think that's not bad: then a visitor can give his/her own meaning to what a star for her/him has to say.