What is the low hanging fruit of things we could be doing to improve society?

Of course there's a single explanation: in a democracy, everyone gets a vote.  Not 'everyone' knows enough about these subjects to understand why the experts are right.  Moreover, in a representative democracy, there is a second layer where entrenched interests - who guard virtually everything you listed - get to bribe politicians to go against the preferences of their constituents.

I do have to ask about one issue on this list: regarding free trade.  The argument has been made that China applies indirect tariffs to U.S. made goods.  Specifically, by either (1) outright banning entire categories of good (no tariff, just ban) such as the Google App Store.  (2)  Subsidizing heavily companies making a competing product.  So, for example, a U.S. smartphone chip vendor can technically sell their parts on the Chinese market without a tariff, but the Chinese domestic competitors to them get billions of dollars in subsidies and thus can price a comparable part for even less.  

I don't know how to deal with this other than the obvious, which is to fight China's implicit tariffs with explicit tariffs.  (which is what is being tried, albeit with a great deal of complaining)

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Yeah!  I don't want a surgeon who's wasted YEARS in supervised (and unpleasant/difficult to be sure) conditions.  Let them pass the test and pick up a scalpel!

A better reform would be "do away with occupational licensing entirely for many non-critical professions.  For those with high risk, replace it with liability/insurance and reputation mechanisms (which will end up looking like accreditation, or they will be unable to get insurance, but there's at least a chance at diversity of types of accreditation)".  

I have a personal belief that a lot of low hanging fruit does not get picked because of we have masses where each benefits a little vs smaller entities with a lot to lose, such as drug companies wanting smaller enforcement. As such the invested minority can outlast the majority in terms of preventing these changes from becoming law.
Do you see other factors having more significance? Further, can we avoid these impasses?

Why are my appliances allowed to be designed to become landfill? Why isn't there the digital equivalent of putting stuff you don't want on the street for people that do want it?

But there is that digital equivalent, it's called ebay and Craigslist.  I have purchased a used washer and dryer for $100, both worked for over a year after.  I sold my entire desktop PC for parts on ebay a few months back, receiving $600 for 5 year old hardware, and I put the unsellable metal case on the curb and posted it to Craigslist.

There's a couple of factors here.  One, due to the time value of money, a cheaper appliance now may very well be cheaper, even if it only lasts 5 years, than a more expensive appliance with a longer lifespan.  Another is that consumer goods are moving over time, as factories get more efficient, to a model where the cost is the (raw materials + manufacturing cost + IP license).  Once manufacturing cost trends near zero - something that machine learning driven robotics should allow in the Soonish future - you are left with raw materials + IP license.  Turn in the old one to be recycled, and that just leaves the license.  Prediction: the stable way it will work is that products will be rented, not bought, and when the hardware wears out or deprecates you get a 'free' or minimal cost replacement.  Lower end goods will have a very cheap or free license (open source) and will cost almost nothing, although they will be years behind the state of the art.  Present example is the raspberry pi, which is a computer that would have been decent 15 years ago for as little as $5.

Why don't they put nuclear reactors underground?

I found this paper, but I think the reason here is simple.  There are many possible improvements to nuclear reactors.  Each has a development cost.  And a development risk.  And nuclear is already too expensive to be viable, probably from now until the singularity*.  So there is simply not the market scale to develop any substantial improvement to the existing examples that we have now.  Scale matters - if it costs say the cost of 10x nuclear reactors to design and debug a new form of nuclear reactor, and you earn 10% profit on each reactor you build, you need a market size of 100 over say 10 years or it isn't worth the investment.  Right now the market size for new nuclear reactors is 2.  

Why is so little of my house designed to be easy clean and filth repelling? Or for that matter, easy to alter and upgrade? Why isn't there such a thing as house lego where you just have a base with a bus of utilities on which you can just arrange some rooms as required?

I agree with this but I think that the fundamental problem is local building code and permitting systems.  If the system were national, then lego-like modules would have a market because it would be worth the investment to build the factory and distribution system to make them cheaply, given you could use the modules anywhere.  But here local committees might vote down the modular house simply because the logical modules are rectilinear, and the resulting structure would be 'too boxy'.  Or because it has electrical runs with the wrong kind of wiring.  (most places, romex in walls is ok.  Chicago: conduit only)

*definition of the singularity : self amplifying cascade of algorithms and AI hardware, with the endpoint of intelligent systems that are asymptotically limited by physics.  At which point, you can ask/command such a system to develop a new nuclear reactor and it'll get done, and be nearly perfect and free of error, at minimal cost in time and materials. 

Why are my appliances allowed to be designed to become landfill? Why isn't there the digital equivalent of putting stuff you don't want on the street for people that do want it?

There are digital ways to do that depending on location. In Berlin where I live Ebay Kleinanzeigen/Craigslist/A facebook group for that purpose are all ways where you can give away stuff for free if other people want it. 

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Yes.  Note that the general idea that "California pays the most but once you factor in state income tax and housing costs, it's about the same" has been true since about the late 1970s.  

I agree with you completely, just:

       a.  Hard to see how it's going to change if it hasn't changed in 40 years

       b.  Competing jurisdictions are a thing.  Theoretically some other city elsewhere will gain a comparative advantage if they have the right building codes and gain a critical mass of tech companies.  And it will turn Bay Area into another Detroit or maybe just another New York City. (NYC, while still viable, is mostly stagnant with ever decaying buildings and ever rising fake rents)    

It's not rational to think it will happen.  I agree nuclear has advantages, but it doesn't come close to penciling in.