I would also like to see a demo with at least an SP level that does not feature in the finished game. I liked this about Half-Life's demo - it showed off the features and some of the enemies, but didn't give anything away from the full SP experience. When I played JK and came to the fueling station level, I already knew what to expect from it, so it was not quite as enjoyable in the context of the full game.
I'm not sure it matters as much with MP levels - a couple of actual levels from the game would get everyone up to speed on at least a couple of maps in readiness for some true frag-fests when the full game ships. A practice bot for MP would be nice in the demo too, for all us 56kers with serious lag issues. ;-)
Of course, I think they should focus all of their attention on the full game at the moment, and only release a demo just before actual release of the full game. I think we've given enough feedback in these forums for the people at Raven to chew on already.