What exactly WAS this game supposed to be? And should I be upset I can't get it anymore?

It's an application in which you can develop games and other programs using a programming language called SmileBASIC, which is based on BASIC. You could then publish these programs for others to download, or download other people's programs. It's rather powerful, and there's a lot of cool stuff that people have made!

Yeah what Oscar said. Don't worry though if you want it I'm sure it will be back. Theyre working on a WII U version anyway.

I need this.

They're patching the bugs that led to it being pulled so it should be back up in a relatively brief timespan.

I HOPE. I need this!! I'll make a kagerou project game!