Leave zombie threads in their grave! Have respect for the dead!
With that said, from a personal standpoint, my only gripes about my K10 (I use this thing professionally, and I use it alot) are:
The frames per second: 3? It's acceptable, but just acceptable.
Low light performance: Could be better. But in Pentax's defence, the K10 doesn't doesn't apply smoothing, whereas everybody else does. This smudges details.
Advanced tracking and focusing: Maybe I'm expecting too much out of a mid level, two year old camera. Also, I have no practical experience using the D80 with sports, so I can't personally compare, but the K10 doesn't have any sort of advanced predictive focusing algorithms which some of the newer high end cameras do. I really wish it had that.
Future of the KMount: Is it secure? I have no idea. Pentax/Samsung release cameras very seldomly. This worries me personally. I don't care if it's sooner or later, but I want to know that there will eventually be a K30.
Everything else about the K10 (and by extension the K20) is golden. They're built well, and I'd argue that the majority of lenses have an Image quality that either matches or beats anything from Canon, Nikon, or Sony. I hear alot about people using adaptors to mount Pentax glass on Nikon and Canon bodies, but I never hear about the inverse.
I've gone out and done photography in pouring rain, -35 degree winter and blowing desert, and it's not given me any trouble. I've captured sports, done macro, done weddings, studio shooting, street photography, night exposures, all of it. And this thing's been a reliable tool through it all.