I'm referring more to work space efficiency, not so much your appliances, although those matter too. Make the room where everyone gravitates and congregates the most user friendly. Daily living revolves around the kitchen so concentrate your budget dollars there if you can only improve one area of your home. Why? Because easy use and convenience ensure safety and control, therefore protection from cuts, burns, slips, falls and drops.
Because kitchens are involved, I'm going to divide this post into cabinets/counters and work space, followed by appliances and fixtures. The kitchen provides a prime example of how Universal Design done well isn't noticeable, other than visitors noticing you've got a really sharp kitchen!
You need room to maneuver while multi-tasking so spacing buffers are a priority. Ensuring a 60-inch turning radius (5' X 5') within the primary prep area is a must and can be achieved in many ways, including using a roll-away cart or island on caster wheels.
You'll also want adjacent space surrounding major appliances like the refrigerator and dishwasher so you'll have enough room to place items you are transferring. Notice in this picture the knee space under cooktop. They make cabinet doors that hide this so it's not visible until needed.
In general, you'll want 24-inches minimum length and depth of counter space adjacent to any major appliance, such as a refrigerator (so don't place in a corner), as well as 48-inches clearance out and away from the doors to allow enough maneuvering room with the door(s)/drawers open.
What I just wrote might be hard to visualize so another rule of thumb is to double the amount of walking space you'd require to allow enough area to move in a wheelchair (i.e. double walking clearances). Another typical standard for open floor space is to maintain at least 30 by 48 inches for approaching an appliance from either the front or side (e.g. side loading dishwasher).
Remember counter tops and cabinets don't all have to be the same level as in standard construction. Any contractor telling you it's more expensive to vary the heights just doesn't want to do it and has decided to charge more if you insist. Counter top heights can range from 28 inches (for kids or those who prefer to sit) to 40 inches. Just be sure to have rounded corners and avoid sharp edges.
You can get real creative with cabinetry to limit reaching, stretching and stooping. Set upper wall cabinets three inches lower than conventional height, slide open doors on lower cabinets to reveal knee space (under sink, prep area or cooktop) and don't forget entirely removable base cabinets. Other cabinet features include adjustable, rotating, pull-down, or full extension shelves and task lighting.
Other considerations and recommendations:
I'll end here and repeat to start the next tutorial about appliances and fixtures that, where practical in the kitchen, allow for sliding rather than lifting packages, pots, pans, etc. Reaching and lifting are stressful and reduce control so you want to design and position cabinets, counters and appliances to be as effortless as possible. Ideally you'll be able to accomplish all aspects of food preparation and cleanup both seated and standing.