One of the most annoying things in a game is just starting a level...and before you have a chance to familiarise yourself you're immediately attacked or hit a death block.

In Brandon's game Diamond, in one of the levels you immediately have to swerve left at the start of a level in order to avoid colliding with a 'death block'. Hit it, and you lose a life and start the level again with no warning.

It's a good idea to give the player a good look at the level before he or she starts to play it. This way, the player can orientate himself, and work out a good way to beat it. If you don't want this to happen, at least put a 3...2...1...Go! countdown in. You just need to activate the groups in the right places.

You should do the same with the end of a level. Disable all the groups, play some cool music, and display a 'level complete!' message. Or a 'game over' message if the player lost. It looks far better in my opinion.

Lastly, you should always have a pause ingame. Just have a counter and dis/activate all the groups needed as necessary. You could put a pause menu in a group and use that through the group.

These are some of the things commonly left out of click games. But they add a lot to it if you include them. I'm going to stop writing now, I'm feeling slightly brain-dead.