Sofar it seems like a more casual game, so I would encourage you to look for ways to get first-time users into the game with fewer button clicks. I thought world of goo did a great job with that.
1. since all puzzles are locked except the intro puzzle, maybe it should skip the stage select screen the first time?
2. achievement and challenge info isn't really necessary for the casual / first time player, so maybe skip that the first time too. (world of goo's system was a little excessive, but is closer to what is best, imo)
Perhaps tabbing out should pause the game?
Locking my computer (ctrl+L) and then returning to the game the screen didn't refresh (all black)