If you browse while logged in to an account, there is a setting there to alter quality based on connection speed. Not sure if that is the default setting.
Edit
Settings listed:
Video Playback Quality[changed]
Choose the default setting for viewing videos
Choose my video quality dynamically, based on the current connection speed.
I have a slow connection. Never play higher-quality video.
I have a fast connection. Always play higher-quality video when it's available.