Small touches can also help unify two separate but open spaces. In our feature rooms, the cabinet hardware, Delta sink and faucet and canister lids are all in a chrome finish. This element is brought into the dining room through the silver frames on the artwork and even the chrome rods that suspend the large light fixture over the table. Keeping little things like this in mind when planning your decor will help maintain continuity between the two separate rooms. the desk sits in a dormer, you can paint the inside walls of the dormer section a different colour than the rest of the room, for example.)
If the desk is just floating around on a large wall, you could create a partial wall along one side of the desk with a decorative screen that co-ordinates with the rest of the room. (Covered in fabric that matches the bedding, for instance.) You could also use tall house plants to create the illusion of separation. If the desk is not part of a matching bedroom suite, paint it in a fresh colour that co-ordinates with the rest of the space rather than trying to make it match the suite. Hang a large painting above the desk to give it focus.
Spaces like rec rooms that have more than one function can benefit from these techniques, as well. Visually defining each area will keep the entire space from feeling chaotic while maintaining an open concept.