I had bought an earlier build Soma Spirits to a convention about a month ago and it was interesting to watch people's reactions. Every single person found a passable tile that was supposed to be impassible. Every. Single. One.
Everyone also got frustrated with an encounter that kept respawning and getting in the way of solving a puzzle, and I ended up removing it shortly after.
Live demos are really great for seeing how your players are interacting with your game's design. And you can tell just by their facial expressions and body language what you're doing right and what you're doing wrong.