So when you have a ~5-7 year old Apple Cinema Display monitor that looks something like this:
and you upgrade to Mavericks (MacOS 10.9) or Yosemite (MacOS 10.10), you will lose the ability to adjust the brightness on the thing via the "Display" Preference Pane. In other words, the brightness will be stuck at whatever level it was at when you were on Mountain Lion.
And this is because of a conscious decision by Apple to obsolete these things to motivate you to throw perfectly good monitors into the trash or recycling bin 🙁 (and buy new Apple products, but of course).
So, how to get your brightness controls back in Mavericks?
Best Answer
There are various threads on discussions.apple.com, but the solution I found on MacRumors I will not repost the solution there, since it's likely to be deleted since it's definitely a bit of a hack.
But one that will extend the useful life of your Cinema Display for at least a few more months (if not a year or two):
"Running KextBeast" requires registering (i.e. potential spam) to download the KextBeast app.
Here is how to do the same thing in the Terminal:
Reboot and voila! You will have brightness controls with your "Vintage" Cinema Display.
I hope this helps somebody else out.