I have 3 sudoku apps on my phone; I can tell you what you are missing: scratch marks. At higher difficulties, it is absolutely necessary to be able to note eliminations or possibilities within a square. Without that, the best you can hope for is casual sudoku player, but soduku is by nature not a game for casual players.
This is exactly true. Soduku players are attracted to words like "difficult," "hard," "impossible," or any other challenge.
The level-up format sends a very strong signal that there are tons of "easy" puzzles and like many of these level-up games, it is going to take a while to get to "hard," and that is a tough investment to make when you are 99% sure the payoff doesn't exist.
It is a nice-looking game, but I don't get the impression the creator is someone involved in the soduku world or understands the market too well.
Think of soduku like you'd think of crosswords. Crosswords have a very quick ramp-up. Those that do well early and like it will quickly move on to the hard stuff, only bothering to do Monday and Tuesday as a speed-trial if they bother at all. They use ink instead of pencil and they get very upset at poorly designed puzzles and bad clues. In other words, you are dealing with a highly finicky crowd.
