My wife and I put in 165 miles this weekend, which gave me and my butt a good feel for the ST. Having ridden a 907 for 7 years, I really got a good data point to compare the two. Here's a couple things that differentiate the two:
Handling - The ST2 is marvelous on cornering. The 907 was always a push to get into and out of the corner, even on a new tire. The 907 also 'hunted' a lot on straitaways. I never seemed to have a line, but was always adjusting. The ST keeps it's heading all the way to the horizon
Chassis - Some of the above applies, but the frame of the ST is like a rock. I now think the 907 was flexing a lot in cornering, making mid-corner adjustments sometimes frightening.
Stopping - for whatever reason, it seems to me the ST is harder to stop, which actually makes little sense. I appreciate the non-abs design, and the geometry of the braking systems seem comparable, but I recall less force being needed on the 907. The ST being 40Lb. lighter, and steel lines to boot, again, I haven't figured this one out.
Power - I was actaully surpised that the ST seems to want more revs than the 907. While the extra 40cc is felt, and again appreciated throughout the band, I used to cruise on the 907 at 3.2-3.5kprms. The ST likes say 3.7-4.2 to stay smooth and ready for the punch. It definitely has more hit when called upon. I guess I expected the inertia of those larger pistons to want slower speeds, but it ain't so!
Vibration - From the previous comment, that 'hit' comes with a lot more vibration throughout. Maybe the Chassis is reflecting that to less rigid parts, and maybe I just don't have the FI completely dialed in, or maybe it's just the extra hp, I don't know. The 916 and 996 I rode way back didn't leave me with the impression of having excessive vibes though.
Comfort - I loved my 907. It was the first road bike I was ever truly comfortable on. The ST actually feels more aggressive in the saddle than the old Paso, and higher up at the same time. I felt more 'in' the bike than 'on' it with the 907. While there's a lot more competence with the ST overall, maybe that slightly lower saddle height worked better for me.
All that said, I still wouldn't go back. I love the ST too. And soon it too will be RED!! (-it's black now, and I never advocate a bike be the same color as the road!!)