Naked people carvings.
(found on a blog site here.)
So as you may have guessed, I am a fan of actual design or realism in sculpture. Granted, the sculptures I've made (save a few) have been cartoons or fantasy, but I still took the time to plan out my work and design it.
Sculpture in the real sense has logical thought. After all, you can't make a 40-ft wide sculpture stand up on one 4-in wide stand. At least, not without some thought about the physics behind it.
As for materials, pleh, take what you can get. I personally have a bias against using metal, though, simply because only metal sculptures can withstand the elements, and those types of things are EVERYWHERE (regardless of actual quality). I even count origami as sculpture.
So sculpture, from a classical standpoint, is carving, molding, or shaping materials. Regardless of the materials. Regardless of the subject matter (although that's not always a good thing).
(Found here.)