Well I'm not sure of that but I think the choice of chords and style of guitar player can have a noticeable effect on the overall feel of the session but I suppose to a lesser degree everyone playing puts puts in a bit of this too. The two Irish sessions in Norwich for example have a different base of regulars and the sessions have quite a different feel to them.

Round here, not that I've seen him in a session in ages, I might mention Adrian Lever on guitar. I find he often can come up with "somethings" (don't ask me what he does) that get me thinking "wow what was that, where did it come from?" - unusual and unexpected guitar bits that work well,.

The really, really great to me though are the (rare to me anyway) occasions where a session goes up to that special notch where everyone is just sort as one being swept along by the music.