Have a little tact on your comments
A lot of work and love goes into making a flash game. It's disrespectful to simply insult their work. Artists put in a lot of hours working on a game and publish their work, and its easy to give nonconstructive criticism in just a few second to write only to make people lose confidence in themselves.
Flash artists aren't born good, they build their talent over the years so that you could play a good game for a few minutes.
The least you can say is why it was bad and say how you could improve it instead of saying "It was bad" or "The graphics were shit."