It is better than my first game. The main things that bother me are that jumping in to the side of something causes the player to freeze. The player should probably slowly slide downwards. Also, blocks are not breakable like in the original mario. As others have mentioned, after finishing the game it causes flash to freeze and then lockup firefox.
It is good to hear constructive feedback and newgrounds is a good place to beta test your skills. Unfortunately, if you want to be a flash game developer, you will need have thick skin because many people will be useless ass holes. However, being a developer can be very rewarding at times.