A colleague just told me I have lipstick on my teeth and I'm thankful. Imagine my embarrassment if no one had told me and I had walked around all day wondering why people were staring at my mouth. After I wiped it off, we had a chat about the etiquette of letting people know that something is askew with their appearance and whether it's appropriate to mention it to casual acquaintances or even strangers.
I think most of us would prefer to know about a small flaw that's detracting from an otherwise great appearance, if all that's required is a quick fix (e.g. mayonnaise on your cheek, a pant leg tucked into a sock, etc.). As long as you're polite and more importantly, discreet, it's acceptable to raise these things with friends, family, co-workers and acquaintances. I also think we have a responsibility to save strangers from humiliation when possible and generally when I've done this, people have been appreciative. In a coffee shop I once leapt up from my seat to tell a total stranger that she had inadvertantly tucked her skirt into her pantyhose and she almost hugged me. I also once politely informed a man in a drug store that he had some green marker on his nose and chuckling, he told me that it was a permanent mark from a mining accident years earlier. Woops.
More difficult to digest, are the pleasent (or not so pleasant) remarks about appearance gaffs that are more difficult to rectify such as the fact that our clothes have become too tight lately, we're overdue for a pedicure or, my personal favourite: "you look really tired today". These kind of comments are not only unwelcome, they are unnecessary. People are well aware of the fact that they have put on a little weight, didn't get a chance to visit the spa or didn't get enough sleep. They don't need to be reminded by a well-meaning stranger.
If you feel you absolutely have to help someone improve their appearance or demeanour but are too chicken to do it you can always visit http://www.nicecritic.com/. A Palette staffer turned me on to this site which allows you to send anonymous e-mails to people with such one-liners as: a breath mint would be beneficial today, you seem to have overapplied your makeup, and please wear a more updated tie. I read all of the possible lines and concluded that, if I couldn't give someone this kind of advice in person, it wasn't appropriate to share it at all but I can see how it might be beneficial.
What are your thoughts on letting people know how they could "improve"?