the amount of people can absorb some of the frequencies.... as in, you'll notice that when you do a sound check in an open hall (as bunny said, resonant), it may sound completely brilliant, but when your audience floods in, it will sound totally different...
(this is so music technology/sound recording at the con....)
you'll also get different rebound rates depending on the venue, ie how much sound is rebounding off objects/walls....
ew, i cant think right now...