I'm really happy with my K-5 but one of the reasons for buying a K-3 is, well I'll just copy paste from the operating manual:
Quote: Easy operation of the mode dial
Release the mode dial lock using the mode dial lock release lever to easily operate the mode dial without having to press the mode dial lock button.
With my K-5 the dial lock button is the one thing that really bugs me and it has now been solved!
Well the new sensor, improved auto focus, AA filter solution and so on can of course also pass as reasons for upgrading.