It's not "the next big thing", as much as it's a lot of little things. There's always been a gap between software and the physical world; the Arduino lets people easily bridge that gap. Though the typical projects are trivial as you suggest, the effect of being able to easily construct such projects is significant.

That said, I think there are other MCs aside from the Arduino that are more interesting today, because they have WiFi or Ethernet on-board, and you get into a much more interesting class of project when you add networking as a core feature rather than something that needs to be bolted on.