I've owned two sport touring bikes, at least they were according to their makers; a 2004 Ducati ST3 and my current 2005 BMW R1200ST. I did a comparison thread on here. Both bikes were around 500 pounds wet, which is 200+ pounds lighter then the Honda ST's. I've always thought of sport tourers as more upright sporty bikes with hard luggage. To each their own.
As far as the "can't define it but know it when you see it", that was a quote from the Supreme Court regarding pornography.
Current- 13 Yamaha XT250 * 05 BMW R1200ST (15/16 WR250R coming in Sept.)
Past- 2x aprilia, 2x BMW, 1x Ducati, 5x Kawasaki, 2x Yamaha