How the question asks, when i make a gui.label with custom font and font size bigger than 20, the gui.label appears very bad.
Here’s an example:
Why do it appears so? Sorry for bad english, but i’m an italian guy 
If code is needed, it’s here:
GUI.Label(new Rect(rightSideCoordinate - 15 - 150, characterInformationBox.height + 10 - 30, 150, 40), "Sevenarth", makeTextStyle(Color.white, textFont, 21, TextAnchor.UpperRight));
GUIStyle makeTextStyle(Color textColor, Font font, int fontSize, TextAnchor textAnchor)
{
GUIStyle gs = new GUIStyle();
gs.normal.textColor = textColor;
gs.font = font;
gs.fontSize = fontSize;
gs.alignment = textAnchor;
return gs;
}
You could use a larger font size, so that each letter is rendered using more pixels in the internal font texture, which should bring out more details, and might reduce the y offset problems. Don’t use a font size too large, as this will zero out the anti-aliasing effect at some point.
Of course you’d also need to scale down your label by the same factor, for example with GUIUtility.ScaleAroundPivot().
EDIT: Or decrease the font size, play around with the factor a bit. Also, I seem to remember that non-dynamic fonts are generally rendered at higher quality, but not sure whether that’s still the case for 3.5
I discoreved why it appears so. Then, with Adobe Fireworks (vectorial image-making application), when i make a label, it put on it Antialiasing x8, instead on unity AA isn’t applied. Is there a way to apply AA on label on Unity?
I assume this has been solved by now, but for future reference I’ll answer it, if someone else googles the same problem.
I had the exact same issue as you, the font seemed to be without AA and I could not figure out why. It worked fine with other objects but not one particular…
After hours of trying to figure it out, eventually starting to think Unity was bugged, I noticed I had accidentally instanced 6 objects on top of each other due to a silly programming error in one script. So the text appeared jagged simply because it was a whole pile of stacked text objects on top of each other. Once I removed the extra five objects everything of course looked just fine.
Maybe your issue was completely different, but my font problem looked exactly as yours. Anyway, might help someone with a similar problem.