Let's begin with weight. Before you purchase your binoculars, make sure you handle them (as opposed to viewing them on the internet or in a catalog). They are going to be in your hand for potentially hours on end, so you will want to make sure they are not unduly heavy. Generally speaking, the lighter the binoculars, the happier you are going to be with them. Anything over 35 oz. is going to feel heavy after a short time. You can ease some of the weight burden by ergonomic straps, but if you have any kind of neck, back or arthritis pain, the lighter the better. However, keep in mind that heavier binoculars will resist movement (remember high school physics and Isaac Newton?) The ramifications of Newton's Law of Motion means that a heavier binocular will be easier to hold steady than a lighter pair.
So you have picked up the binoculars and they are as light as air. Now hold them to your eyes and scan the store.
How do they fit in your hand?
Are they a good match for your hand size?
Is the focus knob easy to reach?
Can you hold the binoculars and focus at the same time?
The focus adjustment should be centrally located, fluid and easy to use. You want to be able to focus quickly and precisely to catch a glimpse of that reclusive tufted titmouse before it flies away. Focusing from close up to infinity should be accomplished in one full rotation or less. If the gear ratio is slow, focusing close up will be fine but your distance focusing will be difficult. If it is fast, you will have problems focusing close up but your long range focus will be great. The best of both worlds is accomplished with binoculars outfitted with variable speed gear that allows you to focus slowly in close and more quickly for distance. If you can't get a sharp view up close and personal or at distances your viewing will become tiresome.
Is the grip the right size for your hand? If they do not fit your hand well, your hand will become fatigued after a short time gripping.
Is the grip secure? Some binoculars have a rubbery grip that is not slippery. Most likely there will be a time that you will be holding them in the rain or in extreme heat...it is important that they are not slippery when wet. Some binoculars like, the Bausch and Lomb Elite series, have grooves for your thumb and feel like they were formed around an actual hand.
So it's light, it fits your hand...now consider balance. Some heavier binoculars feel lighter because their weight is distributed well. Try out a number of binoculars to get a sense of their balance when you are holding them up to your eyes. This is a very subjective feel, but you are the one who is going to either have a pleasant or awkward experience with your "binos".
Binoculars offer a range of adjustments so that multiple users can be accommodated. The distance between the eyes of a child versus an adult is going to vary, so the right and left halves of the binoculars will need to have a minimum and maximum range in width so any individual can use them comfortably.
To test the binoculars focusing ability, hold them to your eyes and adjust the regular focus for your left eye. Then use the diopter adjustment dial for the fine tuning for your right eye. Then use only the main focus control for general focusing. If you are doing some binocular sharing, some have markings on the diopter adjustment for different user settings, particularly if you or others use glasses when viewing through the binoculars.
And speaking of eyeglasses, make sure the binoculars can be adjusted for use with your glasses, if you wear them. Some binoculars have eyecups (their function is to keep extraneous light from your eyes) that can be folded back, rolled or twisted to get a close fit between your glasses and the binoculars. Look for eyecups that have click stops that allow you to vary the distance between eye (or eyeglass) and the binoculars. The term "eye relief" refers to the distance in millimeters from the eyepiece to your eye when you can see the total view. If you wear glasses, you will want a long eye relief. Think 71mm or more.
Keeping out the elements is crucial to the longevity of your binoculars. Some offer no protection whatsoever. Others are waterproof. Take it to the next step and you have weatherproofed binoculars that keep out dirt, sand and water and moisture. Some binoculars have optical tubes filled with inert nitrogen instead of air, that prevent the lenses from fogging internally. If you are making a large investment in your binoculars, consider getting a weatherproofed or marine pair.