I might be the only one, but I'm not really excited about caves. Unless they have a plan to make them more interesting visually, I don't see it being very fun outside of short bursts. With everything above ground, just running around collecting things gives you a good view of the landscape. You come across random people's buildings that you can explore a bit, or rare tree's while you mine, or just find a nice vista to admire their engine. But if you're underground, looking at the same texture for hours, no sunlight, in combination with their lack of an indoor lighting system which makes everything below ground washed out and drab.... it just doesn't sound all that great to me. Also, I like the ability to just run outside my claim and grab a few more resources to continue building without taking 2 hours to mine down to the bottom tier to grab 200 more obsidian.
My fear is that this game is going to fail because it's going to be a cumbersome builder game, and a partial adventure game, not excelling in either category because one handicaps the other.