In Kubuntu (both Precise and Oneiric) the font rendering is really different from Gnome. The ubuntu font is not well displayed because it is always used the bold one instead of 'regular' resulting in a fat interface.
Is there a way to fix this?
Go to Fonts – KDE Control Module.
Set Use anti-aliasing to Enabled, then press Configure... and try different Hinting style settings.
This is because of the Ubuntu font, which has not just Bold and Regular, but Bold, Medium and Light styles.
On your screenshot the font is bold because it's Medium style. Go to
Another solution - use another font. I insist you to try Droid fonts. For this install