It's easy to implement difficulty in a Sonic game. All you have to do is to induce despair. You see, obstacles only harm you in Sonic if you do something wrong. Generally speaking, of course. They are, most of the times, not directly aimed at you - it's your fault if you get hit. This specific type of difficulty is normally more subtle, even if it isn't really easier - just makes the gameplay far less based in trial and error. It integrates calmness to skill, and this is VERY interesting because it's Sonic we're talking about here. You have to build momentum, but the fear that of making the wrong move is doubled because of all the harmful elements that will only hit you if you screw up. The challenge is supposed to be diffuse. Notice that whenever there is a chain of threats, such as missiles or shots etc. can ALL be avoided with a single, simple move. The more half-assed is such move, the more upcoming threats you have to avoid. Difficulty is built by your own mistakes to the point of the unbearable and irreversible. Now, this is where I see the real difficulty in Sonic. The rest is skill tolls, challenge, but these don't bear intrinsic... how can I say it... desire to make you lose your lives and cry before the screen. Yeah. This. So if you can puzzle the player's mind as to make him lose his temper and make the wrong move, you have your difficulty that still isn't cheap. It was the player's fault after all. And this much you can do by testing one's reflexes to the point one breaks down the stream. Objects that come from nowhere are perfect. Well, take that section in Angel Island Act 2 in which a carrier drops bombs. If you keep running steadily, nothing will happen. But if you could add some obstacles that suggested that the player should jump (when he actually shouldn't), like missiles with horizontal trajectory that fly JUST over Sonic's head, the player would perhaps, in fact, jump. Then, a second missile would punish him/her for this and then the bombs would do the rest of the job.