I think it's still to be seen in the long run but so far two main ones come to mind

- Adding a mascot(s) helped immensely for creating an identity / making it not another puzzler

- Tuning the puzzle generation constraints: how frequent 2-,3-,4-,5- size pieces are, how many shared colors, how colored markers are laid out relative to one another in the board, all helped tune the puzzles from not being outlandishly difficult or mundanely boring.