Mouse said

I wonder what the answer would be if they asked what is the most important key to good sound quality, or however it would be written? I think that is good bowing, and very hard to learn. 

Yes, but that's a fairly mature observation. Without wishing to accuse the rest of the forum of immaturity, I suspect the answer would still be vibrato! Too late now to put it forward as a poll, as we have primed everyone, lol!

Besides, vibrato does have a huge effect on tone, that can't be denied. But it needs to enhance something that's there already.

Smooth bowing is necessary, but premature vibrato makes bowing less smooth.

Probably people hear a pro violinist and assume that it's the vibrato that creates the tone. Like AndrewH says, listen to some baroque.

Not everyone's cup of tea, but have some Marais.

Mouse said
I liked that. Did you notice the cellist has finger tapes going all around to the back of the neck, wraps right around.

It's probably a tenor or bass viol or something like that. Those are movable gut frets - you still get them on lutes.

That might interest emily, as with them you can customise your scales.

First I was going to mention the guitarrón, a bass guitar popular in Mexico, but the videos made it seem always fretless, but nevertheless I'm fairly sure any instrument like that can and does have gut frets tied on. Sometimes only three or four, depending on how simple the music is. I seem to have drifted my own thread!