Not quite sure I have shot enough with the K-x yet to be sure (plus my K20 is still in the shop so no side by side...
But this is just a hunch. For me it feels a little like the difference between two different 50mm lenses. With many lenses the difference in the 'look' comes down to a 'feel' if that makes any sense at all.
At base ISO The K20 feels a little warmer to me, not color necessarily, just a tad more inviting... (just a second... my lower Brow Chakra is itching) I may need to spend some time dialing in the K-x, but at low ISO's it still has it's own 'feel'
K-x at ISO 200 + Nikkor 105 2.5
I feel like a crazy Marin County Hippie...
At High ISO the difference is a lot easier to see and to put into words... As far as I am concerned the K-x is in a different league... I am seeing a lot of the "oh but look what my K7 can do" posts poppin up. But with the K20 I always tried to shoot at the Lowest ISO possible, because I always could see the klunky/color blob noise starting up pretty early. I shoot a lot of close in faces so I may be hypercritical of Noise and the NR programs like noise ninja/s and the like. But my K20 tended to stay between ISO 100-400.
With the Kx, whatever magic tweaks they did to that sensor/jpeg engine really do work. It has been said of every new Pentax camera since I have been here "I like the way this one deals with the noise better" "it is a finer grain, almost like film"! Well up until now that has been mostly BS... and the steps in better noise has been baby steps at best. With this sensor/cam it seems more a leap!
ISO 6400 ISO 3200
I will shoot some more with the Helios, since is the lens I am most familiar with on the K20... And let you know how my Heart Chakra feels when I am thru.
But saying that, it is also like two 50mm lenses in that I could probably post the side by sides of the K20 vs K-x and most would not see any real difference.
My Root Chakra is glad I could clear that up for you all~ now I need to go meditate