2009 iMac as 2nd Monitor – Can't adjust brightness
Sadly this problem still exists. My work around is as follows

1) Apple F2 to share displays. The screen is usually quite dark.
2) Apple F2 to switch back to iMAC display.
3) I can now adjust the brightness on the iMac, I set it to my desired level.
4) I Apple F2 back to the shared display I initially wanted.


It is indeed quite annoying and generally a bad experience.