Several people talking about different things here.

I don't really mind if people use lyric sheets or music notation, provided they know what they are going to sing/ play reasonably well.

But it also irritates me if, in a "round the room" session, where people KNOW when their turn is coming up, THEN when their turn comes, they start thumbing through the thick folder for the song they can't find, have to get out of their seat to retrieve guitar from somwhere else, adjust seat position 3 times over, adjust capo 3 times, then still decide they've started in the wrong key, re-start, give a long explanation, etc, etc, etc.
Oh and yes, Acorn4, no flicking through folders while others are singing (or in the present tech age, scrolling through IPads, tablets and mobile phones - and PLEASE, no texting or Facebooking/Tweeting!) just do your fellow-sessioners the courtesy of LISTENING!

As for tuning instruments, I would do mine before the evening starts, and make any fine adjustments during applause, before my turn comes round- if you are using an electronic tuner thus should be possible in the presence if bacground noise.

But agree with Al, it may be nice to have a SHORT explanation of what inspired the writing of a particular song, or other background to it - but that is a very different scenario from what Jack and others were complaining about. Such an intro should never be longer than the song!