The big story with the K20D over the K10D (at least on paper) are the following:
- 14.6MP versus 10MP for the K10D
- ISO 6400 with the new Samsung CMOS sensor, versus ISO 1600 with the Sony CCD in the K10D
- Live View
- AF micro-adjustment for up to twenty lenses
The AF adjustment is also quite useful as my most expensive Pentax lens (a 16-45mm f/4 ED AL, not saying much) needs a large amount of AF correction (+120 um on the K10D, +10 on the K20D).
The K20D is noticeably slower than the K10D. I suspect that Pentax did not upgrade the processor and the 50% higher pixel count is taking its toll. Although the most user-visible upgrade for me (aside from the Live View and higher ISO) is that auto white balance is far more competent than the K10D. I actually can leave the K20D on AWB all the time and it handles tungsten lighting (and even the residual yellow cast of my 50mm Super-Takumar) with aplomb.