I could be wrong, but I don't think that goes with what hesdeadjim is saying. The point seems to be that player's expect core features to be included but once hooked will gladly pay for more superficial features.
It might be hard to come up with that sort of feature for your game as it doesn't feature tiny people running around and is rather abstract.
I guess hints are slightly less core than levels, but not by much, IMO.
Any ideas for a slightly different kind of level/game play? You could sell that as a pack.
Other than a time-trial mode (which seems to be a 'Flow Free' innovation, so I'm disinclined to copy that outright) and sprite-themes ('halloween pack' instead of letters from the alphabet), I can't think of much at the moment.
I don't know about hints being a core feature though. Flow Free includes as many as 600 free levels but only 3 free hints. It could be that they are monetizing poorly on level packs, relative to downloads, and hoping to catch up with hints.
It might be hard to come up with that sort of feature for your game as it doesn't feature tiny people running around and is rather abstract.
I guess hints are slightly less core than levels, but not by much, IMO.
Any ideas for a slightly different kind of level/game play? You could sell that as a pack.