It is difficult to really pin down what man decor is. Guys are a diverse bunch with a wide range of traits and attributes. Labeling an entire gender with a certain personality is a dangerous habit.
However there are certain things which in our society are stylistically male. Not a firm scientific decoration, these are more a philosophy, a set of intrinsic notions which give us a common artistic playground upon which to work. Some things just seem girly, and some things seem male. It is as obvious as the difference between pink and blue.
Defining male decorating preferences is really a matter of understanding the psychology that we expect from this gender. Male décor is, by its very definition, not female décor, and therefore its essence is going to be comprised of the difference between the minds of the two.
One of the things society thinks of men, is that they are more functional than women. While females appreciate beauty, guys like things that work. In fact common conceptions state that men are actually annoyed by useless decorative touches.
This means that one of the key elements of guy decorating would be to choose function over form. A delicate coffee tables made from flowing strips of stainless steal will lose out to a sturdy work table made of oak.
Another manifestation of this trait is the male trend to decorate using their hobbies and interests. Sports accessories, toys, models, these are the favorite accessories of a manly room. Meanwhile flowers and incense stands, candles and lace, objects which create atmosphere and little else, those are generally considered more feminine in their nature.
Another aspect of male décor is that guys are generally larger than women. This means they need more room to be comfortable. They need to be able to stretch out, and feel like the world around them is proportionate to their size.
One of the most stereotypical pieces of guy décor is the recliner. Often this is the only furnishing which is big enough to fit a man's body comfortably. Able to lean back and stretch out, they can generally support the majority of a tall guy's size, in a comfortable and relaxing way.
Size and function trends collide in other aspects of guy decorating. Men tend to like big dressers, big beds, big shelves, and big drawers. This is both because the imposing size matches them, and because these objects tend to do their jobs better. Bigger containers hold more. Bigger beds comfort more. Bigger dressers are generally stronger and more durable.
Men also tend to like different colors than women. Guys tend to be drawn to darker and more earthen colors. Red, brown, black, and green are traditionally masculine, and in some shades have a distinctly male feel.
While these trends may hold true for some males, every guy is different. What is important when creating a space is that you take the desires and preferences of everyone that inhabits it, male and female, and find out where your tastes overlap. Compromise in decorating is the art of creating a symphony from many different visions.