Field of view is the amount of territory you see when you look through the monocular. Imagine a fence 1000 yards away. If a monocular has a field of view of 372 feet at 1000 yards, you will see 372 feet of the fence. If the field of view is 250 feet at 1000 yards, you will 250 feet of fence. Keep in mind a basic law of optics. As magnification goes up field of view goes down. If you want a wide field of view, keep the magnification down. 
Eye relief is the distance your eye can be from the eyepiece and still see the entire field of view. It is measured in millimeters. If you wear glasses when you observe, this is a critical feature, because you glasses won't allow you to get your eye close to the eyepiece. If you wear glasses, look for a model that lists 14mm of eye relief or more.
Barska 8x42 Battalion MonocularClose focus is the minimum distance at which a monocular will focus on an object. The close focus on some models is actually measured in inches, instead of yards, making these close focus models very versatile. There is nothing quite as spectacular as viewing a butterfly perched a few feet away through a good monocular.
Lens coatings affect image brightness. The best monoculars are labeled as fully multi-coated, followed by multi-coated, then fully coated.
You'll obviously want a waterproof monocular if you are in a boat, around water or in a wet climate, but no real need to go waterproof for normal use.
