As it was pointed out the first time youtube-dl was on Hacker News, our user base seems to be split regarding that behavior. When you copy a URL from a playlist, the URL includes both the video id and the playlist id.
Some people want to download the whole playlist when passing a URL from one of the videos (because otherwise it's hard to get an URL that contains just the playlist). Some other people just want to download the video they are currently watching.
That's why we have the --no-playlist option, which you can use and even stick in the configuration file. There's also --yes-playlist to override the configuration file if you need to.
Why not drop to a "did you want the playlist, or just this video?" prompt?
Side-benefit: force care to be taken to be explicit if used in a script.
Personally, without reading any docs the first thing I did was paste in just a video ID (i.e. not the entire URL) so it seems natural to me that including both would be ambiguous.
It's debatable whether that should be the default or not; compare it to, say, printing diagnostic info by default and requiring a special `--download` option.
One thing I always change from the default behaviour is video quality: by default it wastes a ton of bandwidth and disk space getting "HD" versions. I can understand why that's the default, but I don't particularly care about resolution, as it's a pretty negligible contributor to quality compared to the actual content of the video.
Some people want to download the whole playlist when passing a URL from one of the videos (because otherwise it's hard to get an URL that contains just the playlist). Some other people just want to download the video they are currently watching.
That's why we have the --no-playlist option, which you can use and even stick in the configuration file. There's also --yes-playlist to override the configuration file if you need to.