This example uses the default Embed code provided when selecting the "Share" option beneath a YouTube video.
YouTube provides an option to use the "old embed code", which provides code that uses object, param, and embed elements.
Accessibility of the YouTube player is highly dependent on users having opted in to using the YouTube HTML5 Player. Unfortunately, Google still considers this experimental so until users opt in, the default is still the less accessible Flash player.
The Default Player (Flash)
Both versions provide a similar experience, and both are a train wreck for screen reader users. The interface is cluttered with unlabeled buttons and controls, which are either announced as "default button" or "unlabeled 12 button", "unlabeled 13 button", etc. With both JAWS 14 and NVDA in IE10 and Firefox, I found it to be virtually impossible to control the player.
There is some keyboard support within the Flash player in Internet Explorer, but the tab order is illogical and content that pops up or expands (e.g., the caption menu and social networking menus) appear to be inaccessible by keyboard.
VoiceOver in Safari ignores the Flash video altogether (tested using Mountain Lion).
The HTML Player
If users have opted in to HTML5 and are using a supporting browser, Video #1 uses the HTML5 <video> element. Video #2 is unaffected, and is still delivered using the old Flash code.
The HTML5 controls are fully accessible by keyboard, and use the browser's default focus indicator.
The HTML5 controls are well-labeled for screen reader users (More Info button, Share button, Play button, Pause button, etc.)
If users select buttons that result in a pop-up or expanded content, keyboard focus is taken to the newly exposed content, which is marked up well for screen reader users. This requires more testing though—it seems as if there may be some bugs with implementation of the caption and settings buttons, and keyboard support is either inaccessible or unintuitive.