The biggest mistake I usually see with outdoor kitchens is too much counter top space and too big of a kitchen concept. I'll explain.
Everyone thinks they will cart the food outside, prep around the grill/smoker, and have a cold one while doing it. Two problems present themselves in this scenario:
1) The counter tops are almost always dirty, bc people don't take the time to keep those counters clean. End result - people prep food inside, then take it out to cook.
2) Its too damn hot to stand around outside in Texas for 8 of the 12 months. So again, people prep inside, then take it outside to cook.
The most useful outdoor kitchens I see are more of an outdoor space - seating areas, landscaping, built in grill. The whole "I'm gonna treat this outside area like my kitchen" never really works out, IMO. Its hot, its (often) dirty, and people just want to cook out there - not prep, wash, hang out.
If I did an outdoor kitchen, I'd keep the "kitchen" part to a minimum, and spend the money/space on a great seating area, outdoor TV, music, etc. Outdoor entertainment > Outdoor Kitchen.