I upgraded from K-30 to K-S2 very recently, and I agree with everything Adam said. The grip is smaller and more blocky, not as contoured, so it does take a different formulation of my fingers to hold. I'm a woman, so my hands are smaller than the typical man's, so that's probably going to help me out. I'm still not used to the grip, but the balance/weight of the camera is excellent. It is overall less "superfluous" in design elements than the K-30, which may be slightly less aesthetically pleasing, but it does make a nice bundle for holding. The buttons (especially on the four-way controller) are "flatter" than the K-30's, and sometimes I find myself "missing" the button, or I think I'm pressing it, but really I'm just pushing on some non-button part of the back of the camera.
The viewfinder seems about the same, maybe a little brighter. The LCD is significantly clearer and smoother in live view. The updated menus are appreciated. The added features of the articulated LCD and Wi-Fi are, obviously, bad ass.
I haven't pushed the images yet, but the RAW files seem very comparable to the K-30's, which I was very happy with.
The shutter may be SLIGHTLY quieter, but not by much.
I've only had the K-S2 for three days, but I do intend to keep the K-30 on the shelf as a just-in-case. I don't see the point in selling my K-30, since it has nearly 70k shutter actuations, and I've dropped it enough that it has significant cracks.