Thanks for the advice, all! While it's fresh in my mind, wanted to report back and call out a few pieces of advice from this thread that were particularly helpful (or that I ignored to my own detriment). Just my observations--hope this is helpful to someone!
1. Demos are indeed hard
I agree with everyone who warned me away from demos at a techno -- it's dark and hard to see, and dancers just want to groove. "If you need a demo for it, the dance is probably too hard" was a great yardstick for my programming, thanks Chuck!
2. Keep calling
Jonathan nailed it -- if the phrasing is at all hard to follow, dancers will tend to swing too long. I definitely found myself calling more than I would in a non-techno contra (by the end of the dances, my calls were all one beat, but I was often still calling at least some moves). Knowing to look out for over-long swings in particular was super helpful, thanks!
3. Choose easy dances
I got cocky in the second half and programmed some stuff that was above the level of the crowd, and had to fall back to Hey in the Barn when a weird box circulate dance fell apart. The advice that many people gave is super sound -- it's harder to see and harder to focus, dancers probably just want to groove etc., and so easier dances are the way to go.
I want to add one more thing I noticed, which is that I as a caller couldn't get a read on the hall because it was so dark. If I'd been better able to see how ropy the previous dance had been, or that a lot of the experienced people had left at the break, I would have been better able to adjust my program. So there's another point in favor of calling easier dances -- not so much that they dancers can't handle it, as a rule, but rather that you as a caller can't tell as easily whether the dancers can handle it.
4. If you can, listen to the tracks in advance!
I worked with Mark Moore (DJ Flourish) from Philly--he's great! Among other things, he sent me his tracks in advance, which meant that I could get extra precise with the dance/track pairing--super helpful because it's much harder to request specific track features to go with a dance when working with a DJ than with a band. Also, I knew how many potatoes to expect for each track, if they were at all fake-out-y, etc.