Had great fun with the demo and completed everything that can reasonably done. I echo the sentiments about Mithril. Perhaps some of the Mithril based artifacts, rather than simply giving you some there and then, can be buffed with multipliers so dens drop more per kill rather than giving you just 1 or 2 there and then. Obviously, the dens health scaling is also problematic, but I suspect you'll figure something out on that for the full release.
This brings me to one of my big questions for this. I've sunk around 180 hours into the demo on browser (mostly idling while I'm AFK etc.). I have also wishlisted it on Steam and definitely intend to pick this up when it goes live. There is however one thing that slightly puts me off, and that is the prospect of starting from square one. I just downloaded the game as you've said it runs better and late game is laggy and I'm right back at square one. It's painful. Especially having played with all my prestige boosts etc. Any way/plans to implement something that can copy a save over or import a save for a full release?