jobel, your edit is correct: the greenlight concept let's you publish a page for feedback so you can gage where your idea's strengths and weaknesses are. I've taken it down now, but the concept page I created gave me some reallygood ideas for what to change, especially the need to avoid having the game look or behave like Limbo. The final version of Umbra is only similar to that original concept because of that initial feedback.
I did have a link on that page to the demo, but traffic dried up after a few months, unsurprisingly. I'm going to provide a trimmed down .exe demo version of the game on the greenlight page when I publish it.
Then, if all goes well and I get lots of thumbs-up, I'll offer early access so a few players can help me bug test. I've not seen the process before so it's going to be a bit of a voyage of discovery....