YouTube has a page that lets you switch to the HTML5 player. If you visit that page in Chrome, you'll only see the message: "The HTML5 player is currently used when possible" and you can't switch to the Flash player. In all the other browsers, the Flash player is enabled by default and you can click "request the HTML5 player" to enable that player.
Here's the same page in Firefox:
It turns out that the HTML5 player is now enabled by default in Chrome and it's the only player you can use, at least officially. YouTube's HTML5 player now supports all the features of the Flash player, including ads and encrypted streams.
If you don't like the HTML5 player and you want to switch to the Flash player, install this extension.