Besides the usual gamma differences and Windows graphics acceleration, gamma tags in QuickTime files can change appearance of files across platforms.
Frantic Films Software explains: "Quicktime has a feature, mostly hidden from users, which is designed to adjust the display gamma of quicktime movies on different machines to compensate for display difference. Deep within the file, there is sometimes a little tag called 'gama' lurking which tells the Quicktime player what gamma correction the file was encoded with. While this is well-intentioned, motivated by the difference in display gamma between PCs and Macs, the Quicktime player offers no way to view this tag and change it.
QuicktimeGammaStripper utility at Frantic Films Software. It's for Windows but there appears to be instructions for Mac too.
Adobe has a technote for strictly After Effects-related issues: QuickTime movies exported from After Effects CS3 are darker or lighter than expected.
Update: this utility appears to be missing from Frantic; I'll look around for it.
Update 2: File found on internet; see QuicktimeGammaStripper utility for Windows.
QTGammaStripper for OS X PPC and Win32 (another gamma stripper app) and Brightness Issues with H.264 QuickTime, a survey and x264 solution by Chris & Trish Meyer.