"Faster browsing on slow networks with Off-Road mode
Off-Road mode saves bandwidth and compresses data while you browse. It will speed up your browsing experience on slow network connections."
Source: http://help.opera.com/opera/Windows/1432/en/viewPages.html#offRoad
Since data is being compressed, that would affect sites like YouTube where videos can't play if they're scrunched like a tin can. :p