It seems that "Use anti-aliasing: System Settings" is actually using a configuration that may be not known to the user. There may be 2 bugs here.



Reproducible: Always

Steps to Reproduce:
In Fonts settings, 
1- Click "Adjust All Fonts..." button and then set, say, size to 11
2- Set "Use anti-aliasing:" to "Enabled" value
3- Set "Configure..." so that 
a) "Exclude range" checkbox is UNchecked
b) "Use sub-pixel rendering:" checkbox is UNchecked
c) Hinting style to Medium
You want here to have the same values as in the picture at
http://docs.kde.org/stable/en/kde-workspace/kcontrol/fonts/index.html#fonts-aa
4- Click the OK button and then click the Apply button (toward lower-right corner) of parent window (KDE Control Module FONTS)
5- Click the Defaults button (toward lower-left corner) of KDE Control Module FONTS window

Actual Results:  
The "Configure..." button is enabled and will indicate (after clicking on it) that Exclude range checkbox is CHECKED with 8.0pt to 15pt selected. This seems to contradict the documentation.

Expected Results:  
The "Configure..." button should be DISabled, otherwise the "Exclude range" and  "Use sub-pixel rendering:" and Hinting style value should all be read-only.

http://docs.kde.org/stable/en/kde-workspace/kcontrol/fonts/index.html#fonts-aa
"
Default anti-aliasing settings are those used system wide by your distribution and labelled System Settings. 
"
http://docs.kde.org/stable/en/kde-workspace/kcontrol/fonts/index.html#fonts-aa

This does not tell me if default anti-aliasing settings as set by my distribution makes anti-aliasing ENabled or DISabled and, if it is enabled, then what are the anti-aliasing settings (like does it exclude a range, if hinting is slight or medium, etc).

When the user chooses "Use anti-aliasing: System Settings", then the user should know (or should be able to figure out), should see, should be able to figure out if hinting style is medium or not, if there is an exclusion for a font-size range. The associated input text boxes should then be read-only.

There may be more than 1 defined, delimited kind of issue in this bug report.

I am using
Kubuntu 13.04 (raring)
Linux 3.8.0-30-generic i686 (32bits)
KDE Development plate-form: 4.11.1
Qt: 4.8.4

Many thanks to Felix Miata for helping me figure out several elements of this bug report.