I'll tell you some of the basic concepts.
Levels are gigantic areas that you can move around in, in a sort of side-scrolling platformer way.
At the top of the screen you'll have a "mini-inventory" with 10 items that can be selected with number keys or scroll wheel.
By pressing I, you'll be able to access your full inventory (around 30 items) and drag/drop items into/from the mini-inventory.
This game will feature combat, NPCs, various HL1-esque "item containers" that you can break, resources, etc.
Oh, and did I mention you'll be able to "build" with resources and destroy objects for resources? You'll also have a "wall hook", which allows you to stick to solid objects.