Graphics are nice. I kinda like the retro look. I'm afraid that's where the good news ends. It gets boring, because there is no real progress. Enemies are too easy and too far apart. You need to have a progression/evolution. other enemies, other obstacles/dangers. Just longer levels is not enough.
-The collisions/boundaries are not correct (you sometimes float on mid air on "ramps" and you lose a heart without really touching an enemy).
- The controls need improvement (for example jumping beside a box sucks) and sometimes get stuck, especially in corners.
-There is also no visual feedback when finishing a level or losing a heart. Something needs to happen (sprite animation, window shaking or turning red... something).
- Some sprite animation would be nice.
I had to try 3 browsers to get it to work.
- Safari black screen after clicking play.
- Firefox needs correct window focus because when you press space it scrolls down and you lose the game window
- worked in chrome. didn't notice any lag, played about 4 or 5 levels.