I've seen and participated in some very interesting discussions about session ettiquette on other forums, so thought it would be interesting to get some other points of view here.
 
It seems by listening the points of views of others that some sessions are becoming more exclusive and less inclusive.    I think there is an element of truth in this, but also wonder if it hasn't always been the case.    
 
Some sessions are open to all who happen to turn up, where others take a little bit more effort to break in to.    
 
Of course there is no need for bad manners or unwelcoming behaviour, but some good sessions can be spoiled by intruders who play outwith their ability.  Similarily, some good or professional players can come in and spoil a session by using it as a stage to showoff without any consideration of the other players.
 
Another factor which seems to cause unnecessary divides at sessions is the rivalry/ intolerance between the singers and the sing-nots!  But perhaps thats another topic altogether