I didn't know a whole lot about game development when I released the first Filler (some would say I still don't), so when I set out to do a "sequel," my main goal was to take the original game and just polish the hell out of it. The original game was only tuned through Level 15 (I never thought anyone would make it past that... they got up into the 70's within hours of launching it), and the later-level gameplay really suffers because of it. Besides having new graphics and slightly more optimized physics, gameplay is now capped at Level 50. From levels 25-50, the bouncy balls increase in speed each level to add a bit of spice. Even now that the game is "done," I think maybe I should've capped it at 25--some folks are still complaining it runs a little long.
The biggest hurdle, though, was how to take a game where "good" players could play one game for 30 minutes or an hour (way too long for a "casual" game) and break it down into more manageable chunks. That's basically what I'm trying to solve with the introduction of the Challenges--most of which allow you to play a single game scenario. I hope you like them!