Why does my QuickLook show text files with a strange font, and how do I make it stop?
You probably have iA Writer installed on your Mac. They recently updated their application to include a Quick Look plugin to show some file types, including .txt, as multi markdown HTML in the Quick Look preview. People have complained that this is intrusive, and the developers have promised a fix in an upcoming update:

http://support.iawriter.com/help/discussions/mac-problems/3711-quick-look-plugin-is-too-liberal-with-file-formats

For now, you can simply delete iA Writer to stop this behavior or manually delete their Quick Look plugin from the app's bundle, as described at the end of this article:

http://brettterpstra.com/2012/03/19/multimarkdown-quick-look-with-style/