First, the notion of 'flattering' dressing, as practiced by What Not To Wear and its ilk, is pretty much predicated on the notion that the most desirable silhouette is a tall, slim hourglass. While this is indeed a preferred shape in our society, this is by no means the only shape, and while there are certainly benefits to looking like a tall slim hourglass, a sane person may choose a different silhouette for a variety of reasons.
Now, I'll focus here on the perceived silhouette rather than the esthetic preferences of the wearer -- mostly because the wearer's preferences are often subtly (and not subtly) shaped by reactions they garner. That is, I think it is perfectly possible for someone to like wearing a femme, 50's silhouette -- but at the same time "I just like it" is not entirely true. Clothes cannot be taken from their context, and our preferences have been shaped by other people and their responses, positive and negative, whether we like it or not. So rather than argue why we might like certain shapes, I would like to discuss on why we may choose to project certain shapes to those around us.
The choosing bit is especially important to me here. Because of course, it's possible to treat clothes as nothing but coverings for your body, and thus just go grab the first pair of pants on a rack and a random shirt and call it a day. But this is surrendering control -- that is, your clothes will still be sending messages, you just won't be in dictating it. Which to me seems like a bad thing, since you cannot opt out from the whole nonverbal communication in a patriarchal society thing.
So anyway. Evolutionary psychologists and the rest of the crowd that believes that 1950's in the US were the pinnacle of healthy and natural gender dynamics insist that hourglass silhouette is the one that men find visually pleasing (blah blah fertility blah visual cues blah we generalize things from the sample of American undergrads and assume their preferences by no means are conditioned blee)
Deb linked me to this post by a fat positive blogger. She brings to light several important points, but I would like to focus on two. First, some bodies are far away from the ideal that clothing won't be able to conceal the gulf -- and yet, they are expected to strive in that direction; that is fat women are expected to dress "thinner" even though they will never be deemed sufficiently thin by the mainstream