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.
If you want to write a game that supports only 1 meg SVGA cards and requires 8 meg of RAM, you can get away with it in shareware. You will have a smaller audience, but you will have an audience. It is more difficult to sell a game with restrictive hardware requirements to a publisher. Publishers have their own in-house standards, and will require that your game must be able to run on their chosen subset of computer hardware. With shareware, you have the freedom to experiment, understanding that you also have the freedom to fail.