And some more...
There is a major difference between the sound quality of the headphone socket and the headphone connector in the docking port (as used if you have the radio remote). The dock headphone connector is *much* louder - unfortunately, there is a lot more hiss on it than on the headphone socket. There is also quite a bit of random noise audible in the hiss on the dock connector - the 5.5G had hiss, but it sounded like pure white noise. This is more "noisy" hiss, for want of a better word, but it is at a similar level to the headphone and dock connectors on the 5.5G.
So:
5.5G - loud, hissy headphone socket; loud, hissy dock headphone connector
6G - quiet, non-hissy headphone socket; loud, hissy dock headphone connector.
There seems to be a trace of distortion on the dock headphone connector on the 6G as well - the headphone socket sounds (at the same volume) both less hissy and less distorted than the dock headphone connector. The headphone socket sounds good overall - the dock headphone connector sounds less good, to the extent that I probably won't be using it any more!
I'd guess that the dock connector is optimised for a high-impedance load, like an amplifier, (unlike the headphone socket, which looks optimised for low-impedance) and it sounds pretty good through my Denon AVR3805 - no hiss, no distortion, and not a great deal of volume unless the iPod volume is set to 100% - pretty typical for a portable device.
You can't get video out of the headphone socket on the 6G any more, but if you have the old universal dock, you can still get composite or s-video out of the socket on that. Watching H264 QVGA material with 128kb/s AAC audio, I was pleasantly surprised by the video out - without comparing back-to-back, it's difficult to be certain, but I felt it was actually a touch sharper than the video out on the 5.5G.
So, thus far, the good points outweigh the bad - the poor quality of the audio into a low-impedance load from the dock connector means the radio remote is a bit useless now, but the headphone socket sounds better than before, and the video is at least as good if not better.