Your biggest consideration will be the size of the desktop. This will determine much of your desk's design. If, for example, you have several pieces of computer hardware but very little floor space (which dictates a smaller desktop), spend some time designing under-desk storage and possibly above-desk storage such as a hutch. If you have plenty of room for a large desktop, you may not need more than a single drawer beneath the top or an organizer on top.
In addition to the standard desktop materials--plywood, medium-density fiberboard (MDF) and glued lumber--an old door also would work well for a large desktop. Lay it on a base made from cabinets (built from scratch or store-bought), mount it on four legs or on trestles or even hang it from a wall; desktops can be quite versatile, and do not need to be directly attached to the rest of the desk to be functional.
Do not feel that you must place your desktop at standard sitting height. Many users prefer to stand while working, and still others like an adjustable top. Make your desk as tall or as short as you like.
Also, there is no rule that a desktop must be rectangular. You can shape a desktop to fit in a given area, whether that shape is an "L", a "T" or even a curve.
While you can suspend your desktop from a wall, it is more typical to use a base. For a writing desk, this base may be little more than four legs (home-built or store-bought), or perhaps a trestle base; trestles can add a touch of elegance to an otherwise utilitarian piece of furniture.
Cabinets are what most people envision when they build a desk. Use store-bought file cabinets or you can build open bookcase-style units for storing books or computer equipment underneath. Add drawers to your home-built units, if desired.
Add a hutch to the top of your desk for the storage of frequently-used books or to get computer peripherals off your main work surface. It can be tall or short, or you can build it in sections. Some hutches attach to the top so they become part of the desk, but you can treat separate hutches as pieces of furniture in their own right and move them around as your needs change.
Do not allow current trends in hutch design (or any aspect of desk design) to limit you. Build your desk based on your needs, and you will create a successful design.