Good trousers are never the defining characteristic of a man's outfit, unless you're a circus clown. A well-chosen outfit should direct attention toward the face and help it stand out in the viewer's mind, and drawing the eye below the waist does nothing to further that goal. Instead, trousers should present as smooth and unbroken a path as possible up the wearer's body; the best trousers will be able to retain their sleek profile whether the wearer is moving or stationary; seated or standing.
On a more practical note, of course, trousers are also where men tend to carry the little necessities of life -- their keys, wallet, cell phone, and so on. Good trousers will have pockets of the proper size and shape to carry a few small items without bulging; loading the pockets and checking a mirror can be an excellent test for off-the-rack trousers. Custom-made trousers can simply be fitted with all the usual daily items in the pockets to see if any adjustments are needed.
Modern dressers should remember that the "fall" of dress trousers -- the distance from the waist to the crotch -- is longer than that of casual jeans, meaning that the pants should be worn higher on the body. Contemporary jeans are often fitted to be worn at the hips, while dress pants should rest comfortably above the hips. A well-fitted pair of dress pants should never be able to slide off the body on its own, even without a belt or suspenders. Trousers are often tailored for wear with suspenders rather than belts, in which case the fall will be even longer and the fit slightly looser. This allows the pants to "hang" on the man's body, which presents a very smooth and flat drape. Many fine dressers prefer suspenders to belts, and often wear trousers with no belt loops at all.
When in doubt, wear a pair of pants with no belt or suspenders and examine the fit. If the trousers pinch or are slipping off, the fit is incorrect. If excess fabric is billowing or "ballooning" anywhere, or if the crotch sags loosely, the trousers are too loose; if wrinkles and bunching appear in the fabric when you move they are too tight. And, of course, if moving or sitting in a pair of trousers is uncomfortable for any reason, you should be asking yourself if you really want to spend an entire day wearing them.