I'd like minecraft better if you could save what you build on your own machine. I like Roblox, but still I think you need to pay monthly to build more than 1 place. It's fun to build stuff and blow it up especially when others are visiting the place and they have no clue what is going to happen. kaboom!
That's how singleplayer mode works 100% of the time.
Assuming you're using a windows computer, paste in this on the start menu search thingy:
Also, for multiplayer, if the server admin is willing you might be able to get them to export your creations to a singleplayer-type save file and email it to you or provide a download link (if so, just put it in the saves folder shown above)
The server I play on we sometimes import and export things to the server that way.
Err, actually, to clarify / rephrase my above statement slightly:
That's how ((the full / paid version of minecraft)) singleplayer mode works 100% of the time