Lighting and what else for a good-looking voxel game?

For outside: atmospheric fog - the greater the distance, the lighter the colors and the lower the contrast from light to dark. Even linear fog will enhance the feeling of distance.

For both inside and outside: mipmapping - more texture detail (maybe even normal mapping) for close objects. That will help to define edges a little better.

For reflective surfaces: specular lighting, particularly for water.

Lighting in caves: that can be problematic unless you can set a flag for surfaces that are inside a cave or hole. If you can do that, then for those surfaces, use a higher power for the diffuse intensity dropoff with distance - i.e., different lighting equations than outside. That will help darken voxels further from the eye and provide more contrast along edges. You could even code the power to be based on distance. I.e., the farther you are from a cave entrance, the more drastic the dropoff - to the point if you're more than several units away from the entrance, the light doesn't "penetrate."

I've not implemented it myself, so I'm unsure of the approach, but there is more color and contrast in shadow than in light. Not brightness, but color detail. Perhaps if you can use a paler-flatter texture for lighted pixels, and more colorful-higher contrast texture for shadowed pixels..

Part of the issue is directional lighting. With purely directional lighting, every vertex, face and pixel receives the same intensity of light, depending on its orientation to the light source, resulting in a flat shaded look.

The light level I mentioned is similar to the flag "inside/outside a cave", but with 8 bit for better transitions between areas with different brightness.

I already have mipmapping, but I want the world to look more "pixelistic" (something like minecraft, but with 32x32 textures on 0.5 meter voxels instead of 16x16 on 1 meter voxels).

Directional lighting is generally not sufficient for attractive terrain. At the very least, you usually want to fake hemisphere lighting from the sky dome, and some sort of ambient lighting to simulate indirect bounce lighting from the surrounding terrain.

Inigo Quilez has an interesting article on faking terrain lighting. Also on terrain fog.

I use some fake directional-based ambient lighting. This is what it looks like in my voxel engine. The idea came from this article on image-based ambient lighting but I kept it more simple and instead am just using the six sides of a cube to gather lighting.

It's a simplified version of what Swiftcoder posted. The sides facing the skydome have different tints of color which change during the day/night cycle, as well as the intensity of the light bounce (how much they contribute to the final color). The sides facing the bottom are less saturated versions of the top colors. I only did this because the voxel lighting will block most of the light from the bottom sides anyways.

With voxels it should be quite easy to make some sort of semi dynamic GI system.

For some easier enhancements I would add scrolling cloud shadows.(simple projective textures). This add large scale dynamic and spatial variance. I also use lower mipmaps from cloud textures for diffuse and specular skylight shadowing.