Canon HF200 -- what's the difference between MXP and FXP? Yes, I know that MXP has a maximum bit rate of 24Mb/sec, while FXP has a max. bit rate of 17Mb/sec. My question really is... what's the visual expression of the extra bits?

It doesn't seem to be sharpness -- both modes are 1080 x 1920 pixels. I'm doubtful it's gamut. I'm thinking it has to do with decreasing macroblocking during motion, and the way AVCHD does it's compression. Similarly to the effect of the bit rate of broadcast TV signals, but I don't know. Which is why I'm asking. ;-)

Or maybe it is chroma. Could the extra bits get us from 4:2:0 chroma subsampling to 4:2:2? Or is 4:2:0 native to AVCHD and can't be changed?

In some types of fast motion, it seems to decrease some of the artifacts. I'm sure there are other quality improvements but they aren't significant enough that many will notice unless they are really looking for them. I never really think about it as I always default to the highest quality setting on anything I use figuring one day I'll appreciate even a tiny difference.