This is my favourite Flash game.
Gosh, I wish I'd come up with this idea. I'm so insanely jealous of designer/programmer jmtb02. What he/she lacks in a real name, he/she makes up for in brilliance.
Without saying anything about the content of the game, however, I do want to draw attention to the structure. As you play, notice how little happens when you die. The feedback you receive is perfectly minimalistic, yet perfectly communicative, and you're immediately returned to the game. There's a place in games, of course, for more elaborate death sequences, whether drawn-out animations or Mario's simple shrug-to-the-camera-and-jingle, but sometimes the best solution is an uninterrupted return to play. Even compared to Super Meat Boy, the delay between death and rebirth in This is the Only Level is miniscule. If you make a mistake, your progress is impeded, but you're not punished.
You must learn from your mistakes and prove your knowledge to succeed, and you are not discouraged from making mistakes. This is the secret to good teaching.
This is the Only Level is essentially a tutorial. This is a game that teaches the player skills, but, more importantly, this is a game that teaches the player to think and act according to its own internal logic. Most games (like most teachers) think that education must be solemn and straightforward. They lecture and they test. They demand perfection, with no room for trial and/or error.
This is utter nonsense.
Play the game, and pay attention to what you're being taught, as well as how you're being taught. Then feel jealous that you didn't think to make a game like this first.