Falls into a few traps
This game isn't awful, but you could stand to watch some Angry Video Game Nerd and learn from his profanity-infused rage. As a developer, I think it's good to see how the players feel about certain game elements, and nobody expresses the hatred felt by gamers better than James Rolfe. Check him out on YouTube and you'll find out how much he hates things like:
1) Drops where you can't see where you're going. When I hit the spaceflight stage, I noticed a large number of points where the high road just ended, requiring me to jump with no certainty of a floor beneath me. This is terrible, especially in a constant-scroll game where you have no choice but to leap to your potential death. The reason this is so bad is that it momentarily takes control away from the player and makes them feel helpless, which means that when they die, they feel cheated. Players feeling cheated is where you run into people hating your game. Contrast Spelunky, in which you are in control 100% of the time due to perfect controls, and never feel cheated through your hundreds and hundreds of failures.
2) Music that drives you crazy. In the dragon sequence, the music is incredibly repetitive, so much so that it becomes maddening. I was glad for the change of pace in the UFO sequence, but the fact that my first gameplay memory is tainted by that soundtrack (same three notes over and over and over and over and over and over) is impacting my decision.
3) Sudden changes of gameplay rules. In the options, I've set one-hit kills to OFF. The instructions say to avoid obstacles or smash them. Throughout the dragon sequence, hitting an obstacle takes one heart, and getting shot takes one heart. Suddenly, in the UFO sequence, I get eaten and instantly defeated by a monstrous plant of some kind that requires a fairly precision jump or a boost to avoid. The problem with this is that up until this point, everything in the game has indicated that this plant will deal one heart of damage; the instant-kill plant doesn't follow the existing pattern of rules, and more importantly, there's no indication that it works otherwise until it kills you. This falls back into that "cheapness" that makes the player feel like the game is cheating.
Despite these shortcomings, there is fun to be had from the game. The controls aren't bad, and while the graphics aren't great, they do a decent enough job of conveying the gameplay. I would advise that the background art is too bright, too similar to the foreground art, and it can become confusing at times. Otherwise, 6/10. Keep up the good work and learn from your mistakes!