Nick,
I'm going to have two bites of the cherry on this, the first list as an operator, this one as a pilot:
1. Comfortable seats, and a liveable environment. No one benefits by getting heat soaked or frozen because the cockpit was designed for a legless dwarf with no sense of hot or cold
2. Ergonomic control layout. The collective hinged behind the pilot's left buttock must be the No 1 backache culprit of all time.
3. A vote with Helmet Fire, for standardised switchery. Even the same aircraft type within the same company can be wired differently. The Military (IIRC) don't do it, for a very good reason!
4. Accessible inspection areas, oil tanks that can be topped up without needing three hands plus a right angled funnel, fold out steps or platforms to stand on without the risk of falling off, and grease free bearings (wish.........). If I wanted dirty hands before going flying, I'd have become an engineer
5. A powerful, comfortable, vibration free, responsive and capable helicopter that can actually carry full load of passengers, plus baggage, with a full tank of fuel, with the airconditioner on during TO & landing, with all the radios that I need to talk to anyone, and a cup holder. That'll do me
PS regarding my "operator's" wish list, note: DoC's set in concrete. I don't need them reduced ('twould be nice), but I do need them to be absolutely predictable.