Words to introduce the feature set. http://people.gnome.org/~markmc/qcow-image-format.html
Words about performance and use-cases.
Smaller file size, even on filesystems which don't support holes (i.e. sparse files)
Copy-on-write support via backing images, where the image only represents changes made to an original separate disk image
Snapshot support, where the image can contain multiple snapshots of the images history
Optional zlib based compression
Optional AES encryption
Options for performance/data integrity tuning