I had an ST for 3 years and I definitely miss the noise and the torque. 17mpg isn't missed, however. I had mine at 302bhp/360lbft torque and it's a shame the Golf takes a good fair amount more cash to get near that figure

Build quality wise? Well, over 40mph the Golf makes a horrible wind noise that sounds like the window is open slightly. The ST didn't make a peep all the way up to 160 leptons.
As far weight - don't folk realise the Golf is actually heavier than the ST? A good 60kgs more.
Driving wise - the ST always felt more confident in the bends, but I'm not sure if the nervousness of the Golf is because of the run flats. The diff helps pull out the bends in the dry, but is as useless as an open diff in the wet. Brakes don't feel as strong either, and seem to suffer more from fade.
The above seems very negative, but I'm glad I've got the GTI now. It's just a nicer place to be and I was fed up of the drama of taking the ST as a daily driver. The constant pops, bangs and high-teen MPG figures were getting boring.