Trajectory is a simple puzzle game suitable for complete gaming novices or experienced gamers.
There's no need for a seperate 'instructions' screen which can put people off, the instructions are included in two sentences in the first two levels. As new gameplay elements are added as the player progresses through the levels these are also explained in a single sentence.
It has a smooth learning curve and the last few levels are very difficult to complete.
At the end of the game players are given a numerical score based on how well they did (quicker is better and using fewer balls is better), an integrated highscore table would suit this very well.
All progress is saved automatically so players can pick up where they left off at any time, even if the browser has been closed.