In some ways, this is a rougher standard than a retail game. When a customer buys a retail game, they have already invested money in it, and they are going to take a little longer to understand the game and learn to play it. When they are downloading a shareware game, they are evaluating it, and deciding not only whether they want to invest more money in it, but whether they want to invest more time in it. With the retail game, since the money is already invested, the time investment is a given. It is very important these days for a shareware game to have good graphics.
Good art is important.
Good music and sound effects, and sound card support are also important. Shareware, like retail, is a competitive market. If you want your program to be noticed, you must make it stand out. It is less important that a shareware game run on all platforms. Shareware allows plenty of room for innovation.