Well... A same number of questions may have been asked a couple of times, but I will tell you.

There are several things that can make an impact on realism.

Photo-realistic textures
Lighting that can bounce from one object to another (works better with photorealistic textures),
texture size/details
realistic water (only with shaders and render-to-texture*)
stencil shadows can make an impact on realism
Physics for cars, balls, etc.

The more detailed and realistic your game looks, the greater the performance impact on customers' computers on which you're targeting at.

Although you can probably make the game realistic and make it down to those who have 16MB VRAM without shaders, it might not be close to realism.

Also, other than graphics, sound, input, gameplay, etc. can affect how good the game is.

I'm going to move this thred to FAQ.