I haven't driven the S3, but I got a brand new loaner A3 from the Audi dealer when my S4 was in for some service a few months ago, and I pretty much hated it.
It wasn't the power. Sure, the 2.0T was a lot slower than my 4.2 V8, but it was adequate power for the car. The throttle was extremely non-linear, the first 10-20% of the travel did absolutely nothing, and then it immediately jumped up, which made it very hard to drive smoothly. The S3 might fix this, I dunno. The brakes were also stupidly grabby in the way that a lot of German cars are these days.
IIRC, the S3 is DSG-only, no manuals. I really want to like the DSG transmission, but I found it annoying in the loaner. The only control that loaner had was to push the shifter forward and back, and they have the direction wrong. Car magazine reviewers often complain about this (back should be upshift, forward downshift), and I'd always dismissed that complaint figuring it's really just arbitrary and you just need to learn it. I was wrong, it really is hugely non-intuitive to have to push it away to upshift. I think the S3 might add steering wheel buttons, but in other cars I've found those to be non-intuitive as well because they're not always in the same place. IMHO it needs paddles that are on the steering column and don't rotate with the wheel.
Theoretically it ought to be possible to remap the shifter direction in software, but there doesn't seem to be any way to do it.
There's something wrong with how the mirrors are mounted, they're too far back in the chassis or something, because the field of view is too narrow. Also, the front doors are too short for me -- I'm about 6' tall, and with my seat in a comfortable position I couldn't rest my arm on the open window without bumping my elbow into the B-pillar.
I'm generally not a fan of the transverse-mount Haldex system used in the A3/S3, for vehicle dynamics reasons. It behaves too much like a FWD car, IMHO.
Other complaints are probably things that the S3 would fix -- the loaner had a stripper, option-free MMI/screen system, which sucked, and the seats were terribly uncomfortable (I think the S3 gets Recaros?)
I'm an Audi fan, I've owned one as my DD for the last 14 years in a row (a B5 S4 and a B6 S4), but I really didn't like that A3.