The bib bodice is something I'm fascinated with. It is reminiscent of menswear during the frontier days, as well as baby bibs. The juxtaposition of youth and masculinity on something as visually feminine as a mini dress makes this design detail a defining element of style, and a great example of simplicity in fashion.
The slight gathering of fabric around the bib cinches the fabric around the bust so as to create a very attractive, and slimming A-line dress shape. This effect also works on tops; consider it an upgrade from the babydoll.
I'd get this dress, but the cap sleeve is one of the most unattractive sleeves available; the lantern sleeve is perhaps the worst - STAY AWAY!