Thoughts on web game usability after viewing lots of 5k entries:
- don't force a mouseclick to start a keyboard-controlled game;
- don't force a keypress to start a mouse-controlled game;
- support the actual cursor keys rather than some random key set;
- avoid javascript 'game over' alertboxes;
- scale your pixel art up—more screen space != more filesize;
- window.resizeTo() and moveTo() are evil;
- don't 'sign' entries with distracting mailto/url links;
- ...
IF it's Flash:
generally, that's needed to ensure that Flash has focus. No focus for Flash == no key presses. The average person might not know that, so it's just a means to guarantee focus.