don't torrent minecraft that's gay as hell.
mostly I'm looking forward to the multiplayer right now. survival mode is cool, but exploration isn't as much of a thing when you don't have other people's structures to find along the way. the current server technology is unpolished and buggy, but it has potential once it's cleaned up; multiplayer creative mode is fun as hell, and survival promises to be even moreso. it can be genuinely unnerving at times, though, digging through the stone and suddenly finding a sprawling cavern with slurps echoing in the distance. it's sort of annoying that you'll hear those noises even if there's a thick stone wall between you and the zombie, but it's made for a few effective jump scares so far and I can only expect it'll be fixed in a patch eventually. these things are coming out regularly now, since notch is working on it full-time as of May.
the survival beta recently added some really cool things. minecarts and minecart tracks were finally introduced, and can be ridden around on. with survival mode's scarcity of resources, though, awesome things like roller coasters aren't yet that viable, even given the /iron cheat.
redstone's been around a little longer, but it has even more potential to be cool. redstone and redstone torces can be used to create basic circuits: redstone can be applied to any blocks to create a trail that conducts electricity to a torch, which is either lit or snuffed out by the current. people have used this to create elaborate fortress security systems, calculators, and other things.
right now I'm hoping the final product includes a final draft of creative mode, which makes use of things like redstone, minecarts, and the new lighting and liquid physics. I can't imagine what sorts of things could be done by a server full of electronically-minded people using redstone, or some other awesome people making roller coasters.
anyone can play creative mode, and that's pretty fun in its own right, but I'd definitely recommend that you shell out and see what's new. buying in during the prerelease applies for all future updates and the eventual final product, and for half the price the full thing is gonna be.
the Minecraft wiki
is an excellent utility for situations if you're just getting started. it's pretty much the only way to find your way around the crafting system too at this point, beyond simple trial and error.