I was working in Valve's hammer software for more than 3 years now (didn't really release anything as I didn't see any of my work to be complete or be worth releasing, though... I've heard many positive feedback from my hammer friends about my work :S), from seeing what it looked like from trailers about an editor, it looked like building from blocks your level (LEGO anyone?) +/- some scripting (I'm total newbie when it comes to scripting, but anyway I HAVE TO learn it, for my future plans and crap like that).
First, when I will get my hands on the map editor, I will create a simple mission "Escape from the room" alike (ofcourse that won't be it's name). This will be easy enough to complete and I will learn basic things about editor, it's interface and maybe some nice little additional things.
I'm already planning something bigger, a story mostly build from puzzles. You as an adept of magic, you are working on an old ritual, collecting ingredients, reading tons of books, etc. etc. I will try to focus on creating "free world", where you can go anywhere you want from almost beginning. (I hope there will be easy way to create level transition and it won't lag so much on bigger areas :O
).
Gosh, I can't wait.