WPF originally had two major rendering issues. One was the lack of pixel snapping support, and another was gamma correction issues during text rendering, particularly for light text on a dark background (due to an alpha correction approximation, IIRC). The two combined led to blurry text in WPF applications.

These were finally improved for WPF 4, since Visual Studio 2010 switched to it and had a near riot in the betas due to the poor rendering in the text editor.