The data issue can be addressed by only having videos
play automatically when the user is on a wifi
connection. This is the default for mobile devices
already, so it's arguably already solved for most users.
[1]: http://www.tp-link.com/en/products/details/cat-4692_M5250.ht...
Inevitably it updates while I'm tethered to my phone (which has a small data allowance).
Anyone who is outside the norm for whatever reason should be able to exert control over their browser, so obviously you should have the option to block videos autoplaying, or downloading at all, in the browser settings. Whether that should be in the settings page or a flags page is what browser vendors should be thinking hard about. Arguably this is a behaviour that enough people want to control that it should be in the settings page.