Like most of us, I started with a very simple aircraft, the C152 and then got into more complex aircraft. Only after having flown an aircraft with a certain feature, one can judge its value.
What's your take on the "complex" features, how important are they, would you ever go back to an aircraft that doesn't have them?
Here's my list:
Constant speed prop: Absolutely vital for traveling distances. Increased efficiency is nice but the biggest advantage is that you don't have to constantly adjust the power setting in turbulent air.
Retractable gear: the single biggest source of airspeed. Modern aircraft designs tend to not have it due to cost but I love it. Another big advantage is that it gives you a speed brake because the gear is not aerodynamically optimized.
Turbocharger: never ever without again. Being able to climb and not suffering from a constantly decreasing climb rate is the most powerful tool in weather avoidance. It also gives you the freedom to choose the best altitude (wind) and get there quickly. Also excellent to know that no matter what the density altitude is, your aircraft will perform at takeoff.
EFIS: good to have but not a game changer. EFIS are more powerful and cheaper than repairing/buying steam gauges so they're a natural upgrade. Apart from the air data computer, there is nothing significant they add on top of a traditional cockpit with an iPad.
Deice: I don't know because I've never had it. I can add full TKS to my aircraft for €40k but I doubt it would be good value. The combination of a turbocharger and not using the aircraft for time critical business travel makes it seem redundant.
would you ever go back to an aircraft that doesn't have them?
Depending for what! For recreational flying on a sunny Sunday afternoon I need nothing from your list and I will not miss it. I really only care for a reliable and well-maintained engine!
For going places at given time, my priorities would be:
Reliable radio including an independently powered spare unit (built-in or portable dosen't matter). As long as you are in contact with someone on the ground, things like airspace and navigation must not worry you in case of difficulty.
Reliable attitude indicator including independently powered spare unit (when flying IFR). Radio&AI is really all you need to get safely back on the ground.
Anti/de-ice. I refuse to ever enter icing conditions with a non-deiced aircraft again (unless the freezing level is at a very safe altitude)
I don't care too much for the rest, at least talking singles and light twins.
Retractable gear: At the usual speeds of light aircraft, it costs you about 10kt (e.g. comparing a fixed gear Partenavia to the similarly powered Piper Seminole). Why do Cirruses and Cessna 300/350 have a fixed gear? Because the extra mass and complication is not worth it.
Turbocharger: Nice to have, but a complication and source of expensive trouble that might not be worth it. My worst engine failure so far was caused by a turbocharger problem. I have flown over the Alps several times (IFR/IMC!) inn non-turbocharged aircraft, including a C182 at FL160. It takes longer to climb, but it is possible.
One thing I forgot - and very high on the list, position 1.5 I would say for every aircraft I fly: A transponder that transmits as much information as it can. I want to be seen by radar and other traffic. "See and be seen" does not always work as well as it should. Even if I will ever fly gliders again (hopefully) I will not do so without a transponder.
For me:
VP & retractable gear - definitely! Perhaps also, because this combo is what I've been flying almost exclusively for years (C172RG / C182RG) and am used to it.
At least on the Cessnas, the speed difference is more like 15kt, so worth it. Frankly, I wouldn't want to do any long(ish) flight without VP and RG if I could avoid it.
On the retract I also love the 'speed brake' effect of the gear, sometimes comes in handy flying into bigger airports when ATC want you to keep your speed up.
Turbo: only flown a couple of times during my HP endorsement, so cannot really tell. Would be nice to have, though, especially when flying somewhere 'hot n' high'.
EFIS - well, pretty much as achimha says: steam gauges and an iPad work for me. That said, I came away really very impressed by the glass cockpit in Jason's beautiful 'micro airliner'!
De-ice - never flown it, and so far not an issue, as I don't - yet - have an IR.
What I would sometimes like is pressurization. Something like a C182 can get high enough to get you across some serious terrain and/or weather, but then you're in O2 territory. I recently tried to rent some portable O2 kit for a planned flight across the Sierra Nevada in CA and couldn't find any (turned out to be a non-issue as I had to change my plans anyway). And then had a C210P next to me at the runup area at KSMO....
It is obviously important for the user of the aircraft to understand what they need, what they do not need, and what they can afford to install and maintain. There is no one right answer, as to how an aircraft should be equipped and configured. It should be appropriate to the intended application.
So rather than considering a wish list of possible configurations, it is more important to know what you must have to accomplish the task, and beyond that, what would be a nice to have.
After years of flying extremely well equipped aircraft, I opted for as simple as I could get, and could not be more happy that I did. I can rent or borrow better equipped for the few times need that capability, but it is surprisingly rare for me. But, I do not fly IFR, or routinely at high altitude. Obviously, if I did, my equipment needs would be different.
After hundreds of hours flying the Aztec and Cheyenne in actual icing conditions, I am convinced that there are very few singles which should be equipped or approved for continued flight in icing conditions. Those aircraft should be flown with a "stay out or get out" approach.
Things like transponders are nice, but again, are made valuable by the operating environment. The annual recertification costs for a transponder system in an aircraft is in the hundreds. if you need it, fine, but if your normal operating environment does not require it, and the airspace is not busy, their utility may not justify their cost and maintenance. I purposefully did not install the second transponder I have in the second plane I have.
I approved a turbocharger installation on a floatplane for a client, as he needed the power to be available for high altitude lake operation. The cost for that installation exceeded $50,000, and 93 pounds of useful load. He's happy with the plane, and it does what he wants, but carrying less, at a much greater cost.
For any aircraft for which water immersion is a possibility, the less avionics, the happier everyone will be. Many times we have pulled a fully IFR equipped floatplane from being upside down in the water. it's a quick way to trash tens of thousands of dollars in avionics, which probably were not need for water flying anyway!
All that extra stuff costs money, maintenance, and useful load. It's fine if you need it, but make sure it's worth it!
My minimum gotta haves: Four point harness, good headsets and an intercom, decent GPS, survival kit, and life jackets or suits for over water flying.
It depends on your average route, but going places is the way to fly for me.
VP & RG:
My plane is not available w/o both of it.
Turbo:
I added a turbo, to my aircraft last year and it changed my flying. I remember a pre-turbo take-off in Lausanne with 34C and DA of 4800ft, it was more creeping up than a climb as the terrain went up. Oxygen is a must and Mountain High is worth its money.
de-ice:
Something I miss badly, but too expensive to retrofit. It costs more than the value of my plane.
EFIS:
I'm fine with steam gauges, but if my HSI will stop working I will get an Aspen.
New points: FLARM:
I have to get a FLARM. I had 2 near misses with gliders (near Stuttgart) last month. They are so hard to spot.
AVTUR:
A non-AVGAS engine would be fab.
Things like transponders are nice, but again, are made valuable by the operating environment. The annual recertification costs for a transponder system in an aircraft is in the hundreds. if you need it, fine, but if your normal operating environment does not require it...
Bush flying in Canada is probably not going to get much safer by installing a transponder (but who knows, if you have to force-land in the bush, your last radar return might enable the rescue team to find you before the grizzly does).
But here in (south-western) Germany, a mid-air collision must be about the most probable cause of coming to grief for an aviator. We have had - and still have! - our fair share of those, the last one dating only a month ago (http://www.schwaebische.de/region/allgaeu/leutkirch/rund-um-leutkirchartikel,-Flugzeugkollision-bei-Kempten-Behoerde-bezeichnet-Zusammenstoesse-als-nicht-aussergewoehnlich-%2382-arid,5439556.html). So for me: No transponder, no fly.
And regarding the cost: Ask the wifes of those guys what they think about it. (Or as they keep repeating every year during our CRM course: "If you think, safety is expensive, try an accident instead!")
OK, so ignoring your list but making my own.
For long distance business type flying in Europe I want (in no particular order):
For bimbling:
So essentially as said by others, it depends.
As for a turbocharger, again depends. Only really important if you are going high or need to use high altitude airports. I am happy with my normally aspirated engine ;)
I am happy with my normally aspirated engine
A turbine is (as the name suggests) a combined turbocharger and combustion engine. One cannot be more turbocharged than in a turbine. How else would you get enough oxygen to burn those 120+ liters per hour? 
A turbine (turboprop or turbojet) is normally aspirated. It's not turbocharged. The power drops off as you climb.
The power drops off as you climb.
And a lot! Ours is only capable of burning 500lb of fuel per hour per engine at 45,000ft while it can easily annihilate 1,500 at sea level :-)
