Power: The basic power rating of your binoculars is represented by two numbers, such as 8x50. The first number shows the magnification, whilst the second number indicates the diameter of the lens. Each influences the quality of the viewable image. Objects lose their clarity as magnification increases, but larger lenses can compensate for this loss by allowing more light to enter the scopes. But at higher powers of magnification, it may prove difficult to hold the binoculars steadily enough and a tripod may be needed.

Weight: Larger lenses tend to be heavier, which can be significant if you'll be carrying your binoculars for any length of time. Many users have to find a compromise between power and portability.
Lens coatings: Modern lens coatings reduce glare and light loss as light passes through the optical system. This can improve brightness, clarity and contrast of an image and make binoculars more comfortable to use.

Waterproofing: It might be supposed that military binoculars should be able to withstand heavy use and a variety of environmental conditions. If you intend to use them in all weathers, consider the value of getting waterproof equipment. Having optics which are 'nitrogen purged' can also help to reduce fogging in damp conditions or when moving between different temperatures.

urability: For other tough conditions, armoring such as rubber coating on the body can protect binoculars for heavy duty purposes.
 

Source:  Waterproof Binoculars,   Monocular Telescope