Have users vote whether an outfit or "look" is fashionable ("in") or not ("out"). Users could upload pics through the web or from their mobile phones and the community would decide what looks good and what should never be seen in public.
Reference Information:
www.hotornot.com - but for fashion...
Potential Issues:
Need a large community to make this interesting and lots of contributors. Also, the only viable business model is ad revenue. Lots of competition for the fashion attention.