I went "Old World" with DH's and mine home offices. My main principles were:

- Plenty of functional storage. Not a standard built-in closet, which in my opinion has the worst functionality, but bookcases, drawer chests, filing cabinets or stands that suit your needs/equipment and can be moved around as your needs and equipment change;

- Large desks that don't face a wall. A desk facing a wall is my pet peeve. It makes me feel like I'm in a dead-end street. BTW, at the company I worked for in my home country, employees' desks never faced walls. The people sat with their backs to the wall, facing the open center of the room;

- Both of our home offices have couches. One's home office is often his/her private refuge. Many famous writers, politicians, businessmen, etc. had couches or lounge chairs in their home offices. This way there's a spot where you can settle comfortably while looking through papers or making notes in a book. Good for power naps too :-) ;

- Multiple sources of lighting: ceiling light, desk lamps, wall sconces or table lamps by the couch, various task lighting depending on your equipment;

- Multiple surfaces for spreading your stuff around. My habit is that I spread my papers and books on the couch in my office. I also have a long 9-drawer dresser. My printer sits on it, and there's plenty of room to put various stuff I'm working with. It's within an arm's reach but doesn't crowd my desk surface.

Some (very unorthodox) examples of offices of prominent people.
Henrik Ibsen's study in Oslo, Norway

Lots and lots of electrical outlets around the desk, one for each computer gadget and extras for phone, shredder, chargers, etc. Keep in mind that some power blocks hog two outlets. Since my office space used to be the kitchen, I left the outlets at counter height so that I don't need to move furniture just to plug or unplug things.

I'd start by making a list of exactly what you need TO DO in the office -- not only items you imagine there, but tasks you want to complete in the office. If every one of us made such a list, we'd probably find that we vary widely. The house we're building is for our retirement, and my list would look something like this:

- An ideal office would be a long, narrow room. This gives the maximum amount of wall space for the square footage -- and wall space in an office means storage space. On the other hand, empty space in the middle of a square office isn't appealing to me. The picture of Carnege's office above looks good to me: Large desk, space to walk but not 'specially large, but I'd replace the sofa and window with a set of floor-to-ceiling shelves.

- Space for one person to sit at desk to use computer, write grocery lists, pay bills -- we never work together, and have no problem sharing one desk -- one comfortable roll-y chair must be available for the worker.

- Want a built-in desk that faces the wall: Shelves just above the desk. I want to have office supplies located conveniently at our fingertips, perhaps mounted on the wall in attractive containers -- a desk that faces "out" can't give you that same amount of "stuff" at your fingertips. I want never to have to hunt for tape, scissors, a good ink pen, stamps and envelopes, calculator, etc.

- The space above the desk must include a large wall calendar.

- Our eyes aren't getting any younger -- need excellent lighting above the desk as well as task lighting low on the desk. With lighting in mind, light colors would be best for the desk area.

- Want to be able to listen to loud music at the desk without disturbing the rest of the house. As such, this should be located at the far end of the house, away from the main living area. Maybe a small TV?

- Place to open /sort /store mail and other paperwork -- avoid bringing paper into the main house, where it sits and becomes clutter. As such, the office should be by the casual entrance - the place where you'd naturally bring in the day's mail.

- Large trash can (stored under the desk) for tossing junk mail and for use in working with crafts

- Charge laptops and ipads ... sometimes need electrical outlets for craft items ... place these just above the desk -- no one wants to scurry around and reach under the desk to find an outlet. I do not want to see a bunch of cords everywhere.

- Print documents occasionally -- would prefer the printer to be located out of sight -- in a drawer? On a low shelf behind a curtain? Try to keep this important-but-seldom-used item out of the way and avoid dust. Space to store paper and ink for the printer.

- Store craft supplies, especially scrapbooking supplies ... and storage must be attractive to look at -- mixture of baskets and boxes covered with cloth? Want a whole wall of storage shelves -- perhaps located on the wall behind the desk.

- Storage of emergency supplies: Candles and matches, flashlights, batteries, etc.

- Desk top should be tough -- able to withstand future grandchildren's crafts or magic markers -- and my own mess. I'm thinking of an inexpensive Ikea butcherblock countertop /no drawers underneath. Also need enough space to "leave out" a project that isn't finished /needs drying time /whatever.

- Need space to store sewing machine, but since it doesn't come out all that often, it doesn't have to be prime space.

- Space large enough to wrap gifts and store gift wrap materials.

- Store a moderate amount of paperwork -- insurance, licenses, birth certificates, investment info -- and this storage needs to be secure -- perhaps a rolling file cabinet that can slide under the desk? This would also include a simple storage system for receipts.

- Store valuables in a hidden safe -- arrange for this to be built into a hidden cabinet. Can be out of the center of things.

- Hooks upon which to hang extra keys.

- Whole thing should be attractive, bright and sunny with natural light, a happy bright print for the curtains, and easy to keep clean -- no fussy materials. Whole space may be cozy -- large isn't important. Space for photographs on the wall and at the desk.

- The floor should be easy to clean and hard-working: Laminate, hardwood, tile. No carpet

- Finally, the office must include a bed for the dog. He is going to follow me in there, and he needs a place to lie down and nap -- otherwise, he'll just bug the snot out of me.

So, that's my list -- you need to consider what YOU would do in your office. Note that I didn't say anything about having a comfortable reading chair, about meeting clients, about needing storage for any specialized work machinery, about needing a sink for messy crafts, about having a private entrance, about having a separate house phone line or modem for the computer, about placing it adjacent to the kitchen -- because those things don't figure into MY needs, but I know that they would be priorities for someone else! You'll note I didn't mention books -- because I have another area of my house set aside for books. Someone else might want a small table where you could play games as a family, or someone else might want to overlook the kids' play area in the back yard. Someone else might need space for two-person projects, or a place for a gun cabinet.

So, really, step one is your own needs.

Get a real filing cabinet. With full-extension drawers and heavy-duty drawer glides. It's a huge, huge difference.