1. Front fastening
2. Ready-made to fit silicon titty (prosthetic, post-mastectomy), but those who still have "both" can wear it too. My FILA sports bra has a kind of cotton-y, wick-y semi-lining at the sides and bottom of the cups and "titty" just slpis in and sits there all day quite happily
3. Options for racer back or regular straps (over-the-shoulder)
4. Colours - as many as possible

My $0.02