Decide on a Look and Feel
You probably know that the term look and feel is widely used when describing the creative side of a software application. Look refers to the game's overall graphical style, color usage, and animations. Feel refers to the usability, as well as the parts of the game that can affect the user on emotional or tactile levels, including story and sounds. But the look itself can contribute to the feel as well.
Deciding on a look and feel for your game should, of course, be related to the intended audience. You don't want to create dark, gothic graphics for a game that will end up on a children's site. Likewise, you probably wouldn't want to have heavy-metal music playing in the background for a game intended to appeal to corporate executives.
If you are unsure of what would be a good look and feel for your game, check out other games targeted to your audience. Note behaviors and operations that you think work particularly well. Studying those games should help you come up with good ideas for your own. (And during the final step, quality assurance, your test audience will make comments on the look and feel of your game, which will help you see how well you hit the mark.)