"I’ve just looked it up and it is 320 by 240 pixels and runs at 25 frames per second. The maximum data rate is 300kbit/s
NTSC and PAL are effectively 720x480/576 so this is less than Standard TV quality not exactly home cinema …?"
NTSC is typically 640x480 @ 29.97FPS, 720x480 is typically referred to as DVD quality.
YouTube recommends 320x240 @ 30FPS.
So YouTube is 1/2 the resolution of SD TV display. (frame rate is similar to film(24FPS))
YouTube also restricts uploads to a 100MB file size limit which forces any decent content @ decent quality to be very short.
As Jeff said, the 10min limitation is truly the only reason YouTube remains only an annoyance.