quvi does not seem maintained (last upstream release in 2013)
and it supports many fewer videos than youtube-dl does.

The difficulty with using youtube-dl is it, by design, does not
provide a way to probe if it supports an url, other than running it
and seeing if it finds a video at the url. This would make `git annex
addurl` significantly slower if it ran youtube-dl to probe every url.

It is possible to use youtube-dl to download arbitrary non-video files;
it stores the file to disk just as wget or curl. But, that's well outside
its intended use case, and so it does not feel like a good idea to make
git-annex depend on using youtube-dl to download generic urls.
(Also, youtube-dl has bugs with downloading non-video 
urls, see for example http://bugs.debian.org/874321)

So, switching to youtube-dl would probably need a new switch, like `git
annex addurl --rip` that enables using it.

Currently `git annex importfeed` automatically tests for video urls with
quvi; it would also need to support `--rip`.

Both of those changes would need changes to user's workflows and cron jobs.
git-annex could keep supporting quvi for some time, and warn when it uses
quvi, to help with the transition.

Another gotcha is playlists. youtube-dl downloads playlists automatically.
But, git-annex needs to record an url that downloads a single file so that
`git annex get` works right. So, playlists will need to be disabled when
git-annex runs youtube-dl. But, `--no-playlist` does not always disable
playlists. Best option seems to be `--playlist-items 0` which works for
non-playlists, and downloads only 1 item from playlists (hopefully a fairly
stable item, but who knows..).

Another gotcha is that youtube-dl's -o option does not fully determine the
filename it downloads to. Sometims it will tack on an additional extension
(seen with youtube videos where it added a ".mkv").
And --get-filename does not report the actual filename when that happens.
Only way I can find to avoid this wart is output to stdout with "-o -",
but that would prevent resuming. Seems it would have to be run in a temp
dir and the file moved out to the git-annex location when done, which would
prevent stuff that operates on incomplete downloads from working.