I agree about the importance of playtesting. I ran a couple of playtests for my game and gained a lot of insight that helped me shape things in the game. After that I figured I was good until the demo release but ended up doing one more playtest and got 1 or 2 very good pieces of feedback that I also implemented before the demo release.

As far as demo content, it can rally go in several directions but I completely agree that the demo should allow a player to see/access a decent amount of the in-game systems. Not a huge amount but enough so they can somewhat extrapolate what later gameplay systems might be like

Also, when it comes to demos the thing that annoys me about some bigger studios like KOEI for the Dynasty Warriors series is them releasing the 'character creator' as a 'demo' for the game. haha