Quick Look support for non-native video files (.avi, .mkv) on Mavericks
I've uploaded QuickLook and Spotlight plugins here: https://github.com/Marginal/QLVideo/wiki . These allow Finder to display thumbnails, static previews, cover art and metadata for most types of video files, including .asf, .avi, .flv, .mkv, .rm, .webm, .wmf etc.
"QuickTime on Mavericks" is not QuickTime anymore - it is "AV Foundation". QuickLook and QuickTime Player are based on this "new QuickTime".
"AV Foundation" comes from iOS, was implemented for Mac OS X too and does not offer extensions (and will never) like codecs / plugins. As Apple says: anything else than MPEG-4 (H.264) is old stuff and must be converted.
In my opinion, it is confusing for users that Apple is still using the name "QuickTime". Because Mavericks still contains the "old QuickTime".
QTKit is a bridge between the old QuickTime and the new QuickTime. It is not deprecated and contains a video converter. This way incompatible video formats can be converted to MPEG-4.
In other words: follow Apple's plan and convert all your videos. Then QuickLook will be available for you.
This has to be the single most annoying change in Mavericks. From what I Understand the new version of Quicktime is not well enough documented to allow this development yet.