This issue is due to the adaptive brightness feature on windows 8, that somehow overlaps with the adaptive brightness provided in bootcamps drivers. You have the fix here:
I have been having the issue of no brighness control when running windows through bootcamp for about 6 months now and have not found any applicable solution online.
While messing around trying to figure it out yet again I noticed that my monitor driver was signed by team viewer...
Sure enough clicking the roll driver back button resolved the issue by rolling the driver back to the Microsoft driver...
So long story short:
Right click "My Computer" click properties
Click device manager (lop left of window)
Expand Monitors section
Double Click monitor
Click driver tab
If the driver here is not signed by Microsot or Apple click the roll back button. My driver version is 6.1.7600.16385 (Driver date 6/21/2006)
Rolling this back instantly resolved my issue. I will now propagate this post to all the threads I saw regarding this issue.
I am running windows 7 x64 Ulti on a 15" rMBP.
Best of luck
SirAce, you bloody rock, sir.
This was exactly what worked. Disabling a PnP monitor brings back bootcamp brightness tab and brightness bezel. Thank you very much. I will post about this solution on my mac blog, referring to this thread.