Apparently the Demo ends with taking the notebook from Biff. There's really not much meat in the Demo then.
Right after that, a Doc-less Delorean shows up with of all things, Einstein alone in it. That's where the story really starts, and it's a damn good story so far.
As for the gameplay, yes it's simple. It's a throwback to the point, click, text sierra adventure games of the past. You find and use items, and try to figure out where to go, and how to accomplish objectives with what you've found.
Not everyone cup of tea, but I think it works really well here.
It's heavy rain, without all the quick time ********, in BTTF universe.