The problem is: KDE has no support for "demi bold" (or semi bold) fonts, while GTK yes.
I was using a font that has a "demi bold" option, which was translated by KDE to bold.
This appears to be fixed since version 4.8, but not an option for Kubuntu users (mainly if you're using 12.04 LTS, as me).
First: on KDE Settings > Application Appearence > Fonts. Mark Anti-aliasing as enabled, click on Configure, UNcheck Exclude range, check Use subpixel, and select Slight for Hinting.
Then do one of the following steps:
Workaround 1: go to KDE Settings > Font Management and disable the semi bold and light variations from the font you're using.
Workaround 2: select a font with no semi bold and light variations, like Tahoma.