I'm so, so happy with the focus on simplicity and really excited to see the rest of the set. To me, Adventures, Empires, and Nocturne just complicate and obscure the fact that most strategic synergies are really elegant and simple. In general, I love cards that introduce one mechanic and make it viable (e.g. Chapel, Scheme, Smugglers, Fortress, Band of Misfits); I dislike cards that tack random effects together, at least when they are conceptually and strategically unrelated (e.g. Watchtower, Horse Traders, on-trash effects like Squire and Hunting Grounds, on-gain effects like Doctor and Herald).
Mountain Village seems quite strong for a village. It acts like an accelerant when you're not drawing deck, letting you replay key cards one shuffle, or the new card you just bought last turn. Once you are drawing deck it behaves just like a normal village (with maybe some mid-turn gain shenanigans), but by that point, it's done a lot of extra work for you.
It has a slight drawback in that it anti-synergizes with sifting, because you are forced to draw stuff that you wanted to discard. But in most cases I agree it seems very strong.
In my playing, it seems the anti-synergy can be mostly removed by playing around it appropriately. So, if you get Mountain Villages and Warehouses in the same deck, what you do is learn to discard a good card with Warehouse whenever necessary. Also this only matters just after a shuffle or if you have already sifted through your whole deck this turn and picked up all the actions.
Even if you get unlucky and have to discard something with Warehouse only to pick up the same thing again with Mountain Village, Warehouse + Mountain Village is the equivalent of "+2 actions, draw 3 cards, discard 2". Compared to a regular village, the net effect of the Warehouse here is "+1 action, +2 cards, discard 2", which is not as good as Warehouse was before, but still fine.
Overall I think Mountain Village is really strong. When you draw it with a lot of cards in your discard, it gets ridiculous, because you can chain all of the Mountain Villages before continuing with whatever action you want. (I tried with Scholar, which is interesting, but not directly a good synergy; you need lots of payload cards also also discard cards or at least don't draw, and sifters like Warehouse are not payloads.)