Here's Mojang's not-quite-exhaustive list of what you can find in this snapshot:
- Redstone circuits are more consistent and pistons should be more stable.
- Added the redstone Comparator block (this replaces the capacitor mentioned at Minecon).
- Added the Daylight Detector block.
- Added a Hopper block (still work in progress).
- Added a chest that activates redstone.
- Added weighted pressure plates that detect item stacks.
- Added redstone block (works as a pushable redstone torch).
- Added a new ore to the Nether.
- Added a Nether Brick item for crafting Nether Brick blocks.
- A whole bunch of other stuff... Try for instance double-clicking items, or drag-placing them, in the inventory.
If you're trying out a Minecraft snapshot for the first time, we have a guide designed specifically to help folks like you, just over there in the corner. Go on, give it a go.