Common Mistake: Too much kitchen counter clutter
Easy fix: It doesn't quite seem like this would fall under "design," but it does if clutter on your counters takes away from your room looking good. Figure out how many items you'd like to hide, and eliminate older things you don't use by donating them. Make room in your cabinets or drawers and declutter your counter to make your kitchen feel calmer (and maybe even larger). You don't have to clear things off completely, but even a small reduction in counter top clutter will help a lot.
Common Mistake: Bad lighting locations
Easy fix: Take a look at your lighting and see whether it needs to be rearranged to balance the light throughout a room (usually three or four points around a room is perfect) or if you need better task lighting in the places you actually work. Don't be afraid to shop around your house to find lamps to re-place, or even to come up with creative mounting solutions.
Read about the other 3 design mistakes and how to fix them here.