So TrendMicro has published a blog post that lists two unpatched vulnerabilities that affect QuickTime for Windows. Worse? They announced that Apple will no longer provide security updates for that software, either. These exploits will continue to exist until you uninstall the software (unless Apple has an abrupt change of heart). Basically, uninstall the software.
OSX users are unaffected. QuickTime is still supported on that platform.
For most users? This shouldn't be a big deal. There really isn't anything that the free QuickTime Player does which cannot be accomplished with VLC. Then again, I'd expect that many of those users (who would also be reading our website) have already moved on.
QuickTime Pro and Adobe users will likely be more affected by this. The formats and utilities that Apple provided are very useful in professional applications. For instance, QuickTime is one of the only reliable video formats (unless something came up that I was unaware of — correct me if I'm wrong) that had an alpha channel for transparency. This allows you to share translucent footage between applications without resorting to some frame-by-frame solution, like a PNG sequence. It is also required to handle QuickTime footage in Adobe Premiere, if you need to collaborate with a Mac user or you have QuickTime-centric hardware.
This is mighty annoying of Apple, but that's a downside of relying upon proprietary software.