Hi Derek,
I'm curious do you mean that in general, text in WPF appears bolder than in GDI or WinForms? We're making some changes in the Rc1 release that may help.
As for specifying varying degrees of font weight, we actually have this ability in WPF. You can set this by using the FontWeight property and setting a value such as SemiBold, Bold, or Black. However, this only works if the font family you're using has these weights available.
The following will work because these font families have a 'Black' font weight available
<TextBlock FontFamily="Arial" FontWeight="Black">Hello</TextBlock>
<TextBlock FontFamily="Bodoni MT" FontWeight="Black">Hello </TextBlock>
WPF will actually simulate 'bold' if a font family does not have a bold weight but we will not simulate any other weights because simulation can significantly decrease the quality of the typeface.
Hope that helps,
Chris