I want to start watching the downloading file immediately, so if youtube-dl downloads video and audio separately I'll be foiled. I.e, I want youtube-dl to choose the best format that has both video and audio.
From the project's README:
If you want to preserve the old format selection behavior (prior to youtube-dl 2015.04.26), i.e. you want to download the best available quality media served as a single file, you should explicitly specify your choice with