First of all, it's still in Alpha mode (before beta) so not everything works properly yet.
Answers on your questions:
It is fun, and it's damn addictive.
There is a multiplayer, and you can host your own server.
It isn't free, right now it costs 10 euros (don't know how many dollars or whatever that is), and when the game actually really comes out, it will cost 20 euros
Those 'green guys' are creepers. They are sneaky, make almost no sound at all, hide in tiny corners where you don't see them and will explode in your face when they get near enough.
There are weapons (not for buildings, and you can't make snowmans that fight ofc).
There are no guns (for now, maybe in later updates).
There are many diffrent skins for anything. You can download someone elses on the forums.
You can make your own skin very easily with paint.
You can play in first person (and you start in it).
You can switch to third person by pressing f5 in game.
About the actual flash:
Minecraft is indeed a stress releaver, with very nice calming music (but also some creepy sounds in deep underground caves), so I'd see you should get it.
Oh, and you also pointed out one of the goals in Minecraft, making rollercoaster/minecart systems. I loved this flash, keep up the good work!
(also, I saw someone walking around in Lances skin in multiplayer XD)
And ofcoarse, there are many funny things in minecraft: The first thing to do is to whack a tree for wood... with your bare fist.