What are the most striking characteristics one finds inside limestone caverns?

Most caves are in limestone, and their primary characteristics are passage forms that show how the cave developed; and calcite formations (stalactites, stalagmites, etc.)

Some have impressive streams in them.

There is also a range of less obvious characteristics & features such as sediments, structural-geology evidence exposed in the walls, and so on.

Probably the one thing most people think of is the darkness. It is absolute, something we never experience in our normal world on the surface; and even very experienced cavers waiting a while in one spot underground rarely turn all their lamps off for more than a few minutes unless they really have to conserve the batteries.