I agree with the Snail that Listening is the key (pun only slightly intended) to a good session. It all works better when everyone is playing the same tune/or song (I've seen the opposite happening quite often) in the same key at the same tempo and the same rhythm.   The result of this all coming together can be quite amazing even if most (or none) of the participants are really good musicians.

As someone who sings in sessions, backs other singers in sessions and backs tunes in sessions, I think the singers have it easier/instrumentalists have it easier debate is a bit silly. The best sessions should have room for both songs and tunes (without words) but there are times where a small group of either tries to take over the night, and this can get very messy.

To get back to Shaneo's original and subsequent post, I think variety is the key to overcoming this. Our local session has been happening weekly for over 10 years now, with (more or less) the same core group of musicians every week. This could quickly become extremely boring (for me at least) if everyone did the same material (tunes or songs) every time. Finding new material may not always be easy, but if everyone brings some new stuff along every so often it helps to keep things fresh and keep everybody interested.