Also, for those of you that already solved most of the challenges - stay tuned. We'll launch more challenges during a big Unix 50th anniversary celebration event taking place next week in Bell Labs HQ in Murray Hill, NJ.
Speaking up here as the person that designed the game at Bell Labs: we'll get rid of the signup wall - it really was meant only to get a unique nickname so we can put your score up on the leaderboard. We like the suggestion of generating a unique bookmarkable url instead.
As for why we don't allow you to type commands into a terminal: the game is as much about solving a puzzle with the basic blocks you're given as it is about shell programming. For newcomers the scratch interface a nice way to explore the kinds of things one can do with the unix utilities (in case it wasn't clear: the game is meant to be approachable to people unfamiliar with unix)
And besides, if we'd give you hackers a full-blown shell, you'd solve most of the challenges in no-time - where's the fun in that? :)