I don't make flash, but i do play games and there was a couple of things i would have liked to be a bit different.
firstly i think the bat should be controlled by the arrow keys, or the number keys for smoother control as the bat kinda drifts with the mouse, it doesn't feel smooth or precise, it needs a feel of more instant control. either that or pin the board to the mouse. But using the arrow keys would restrict it's speed, something that you need in a 'pong' game.
The ball moving in random directions was kinda good, though as a practical gameplay element i think it should revolve around a 'power up' system rather than just randomly taking place, the onus would be on the player to take advantage of the phyics system. Plus knowing what the power up equated too, so you could prepare against it too.
A tournament mode would be a very good edition as well. Like a 10 game difficulty progression, the first obviously being easy, the last extremely hard. I think it would add some much needed depth to the game, a possible online multiplay would be excellent too but that is probably impractical.
The basic sounds were ok, they function well for the game. maybe a few effects for power up collecting, hitting the side walls and a sort of 'score' noise could add to the atmosphere. The music, is good tho, it suits the mood of the game. I thank you for using my tune. I did notice though once it had finished that it didnt loop, there was just silence after that, maybe you should loop it or add a few different tunes, some simple loops and what not.
Graphically it wasn't astounding, but they weren't too bad either, the bats looked good as did the board and ball, maybe a customisation of colours for the bat/ball would be nice and also an addition of a couple of different playing boards, they would just add touches of depth to the overall game.
overall though it's not bad, it needs a bit of work in places and has potential to develop into a very good game. I understand it'd probably hard to implement the things i suggested but if you could i think it would benefit the game significantly.