Atleast they were interested...
I've played lots of open mics where it was an acoustic sort of affair. It definately is a different vibe, you have to rely a lot more on the song I think. Or rather, you have to work much harder at playing something that grabs people's attention with either the chord progression (if you're solo) or the interaction between the instruments/vocals. It's about being interesting without effects.
What makes it really work though is a good croud (like any event). Nothing sucks quite so much as an audience that would rather here a top 40 cd on suffle.