Minor UI suggestions: make the text next to the coins and heart larger, color coded, and maybe place them both above the four keys, next to each other.
Since you've added multiple keys, you could make the levels more non-linear, and force the player to backtrack (obtaining one key is necessary to obtain another key located earlier in the level, for example).
More coins? The coin in the area with the flies at the beginning was fun because it was a challenge to collect. Maybe add more of those challenges, and then display the number of coins collected on the final screen. This adds replay value: players keep trying to collect more coins and get the highest possible score.
Also, recommend a fancier "congratulations" message on the final screen (cooltext.com), and you could use the sparkle effect for "fireworks" -- every 1/2 second, for example, spawn a randomly colored sparkle at a random location on the screen. That could be cool :)