While the idea of WYSIWYG has been around for some years, we are still not fully yet there. One of the problems is that while printers typically have a resolution of 2400 DPI, screen resolution is typically still around 100 DPI, higher resolutions screen typically require good eyesight, as the GUI typically does not scale to the resolution of the screen.
One of the main issues is the scaling of fonts. When vector fonts where introduced, more than ten years ago, the result of rendering the vector at small sizes would typically be of much lower quality than hand-tuned bitmap fonts. Because of this, bitmap fonts typically had priority over vector fonts, and a feature, called hinting was added to Truetype and following font rastering systems. This feature basically fudges the vectors to make them align on pixel boundaries. The alternative is to use full vector rendering and anti-aliasing, but this result in an blurred aspect in small size fonts. Mac OS X typically does this and this rendering technique is also used by Safari for Windows.
While this basically solved the short-term problems, this fudging also made freely scaling texte much more difficult. I found an an interesting article on the issue of scaling and font rastering by the way of OSnews, that basically explains the approaches followed by the different operating systems and the problem they face.