Recently YouTube finished transferring to HTML5 instead of Flash, and as a result, FlashControl plugin for Chrome stopped blocking YouTube videos from autoloading on page open.
This is a frustrating behaviour for many, and the funniest thing that the same FlashControl plugin in Firefox blocks both flash and html5, but not in Chrome. And there is now html autoload blocker for Chrome. There are two workarounds. First install this plugin, to force YouTube load old flash player, not new html5 player. This makes it possible to block it with FlashControl. Another way is to install one of these plugins to stop html5 autoplay on Youtube. They both work and prevent the video from autostart, however unlike Flashcontrol, they don’t prevent video load, so it downloads with corresponding traffic and memory usage.