It seems you are only comparing Minecraft Classic with Atmosphir. Minecraft has a completely separate mode, which is currently called Alpha or Survival, which is what most people love and play in Minecraft, myself included. I could explain exactly what it is, but it would be much easier, and entertaining if I just linked you to a Let's Play session with it.
http://www.youtube.com/watch?v=4bh4EexJO4I
I would recommend you watch the first 5 or 6 episodes.
Basically, Minecraft and Atmosphir are only similar in the fact that you can build.