A key difference in otherwise similar binoculars is the lens coating. Optical coatings give the lenses a blue, green, amber, or red tint. They improve image brightness and contrast, without affecting the color.
Low-cost models have only single-layer coatings, often on just the outside lenses. In better models, all optical surfaces are coated, and the best ones feature multi-layer coatings.
Some binoculars have rubber armor to cushion the optics against minor bumps. Premium models are often waterproof, which prevents dew and contaminants from seeping into the innards.