WTO rules are the absolute basic, no country wants to trade on them. All existing trade relationships we have through the EU will have to be negotiated afresh from a weaker position - we're no longer trading as part of the huge EU buyers club. It's also complicated by the fact that the UK has nowhere near enough qualified trade negotiators. We've contributed a lot of expertise to EU negotiating teams, but as part of a team.
The EU was very good at redistributing wealth from richer areas to poorer areas through regional development funding. Many parts of the UK have benefited from this - the east coast of England, the North East, Wales, etc. The chances of a post-Brexit UK government picking this up are, in my opinion, very low.
International collaboration on medical, scientific and industrial research will become more difficult. Within the EU there were almost no barriers to cooperation with our European neighbours. The people involved could move freely between EU countries and it was easy to set-up a project wherever it was most convenient within the EU28.
We have an economy built on a pyramid scheme, relying on lots of younger peoples working to care for the elderly, and a large pool of employed tax payers keeping the system going. People are getting older and they're having fewer children. The pyramid is in danger of collapsing. Now some amongst the Brexit leaders don't see this as a problem, because their part of the campaign has been funded by a health/care insurance billionaire who'd be rather keen to see the NHS model changed.
In some professions recruitment of UK citizens to the workforce proved very difficult. Nursing and healthcare has had the media attention, but one of the most critical is vets. Not the cute and fluffy small animal vets, but the animal welfare and food safety vets that observe abattoirs - 80% of these vets in the UK are Spanish. Hygiene and welfare standards will drop if there's insufficient supervision - this is on top of chlorinated/hormone meat from the US that Trump would like to export to us.
Opportunity will be reduced. Freedom of movement went both ways. UK citizens have been free to travel to work or study or live in any of the EU27. Because we have a problem with learning foreign languages, and because we have a cultural problem with "foreign" in general, these opportunities haven't always been fully taken up by UK citizens.
I'm one half of a UK-EU marriage. The stress of uncertainty has been crippling. Our plans of dividing stages of our life together between here and there have been torn up and thrown to the wind. I took my opportunities to live, love and work across transparent borders. I resent the choice of those that believe I shouldn't have that opportunity. I am repelled by the casual racism my EU friends are reporting to me.
There are no advantages to being outside the EU. A lot of puff and wind has been blown by people pointing to our financial contribution and vague an unspecified Regulations or Red Tape. As a country we gain back more than we spend. We punch above our weight internationally as part of the EU block. We have a better deal in the EU than Germany.
What we have never done is take the EU seriously. There is nowhere near enough coverage of European issues in the UK media. The general population is ignorant of even the basic topics of European news and conversation. We've just participated in the second largest democratic process in the world, yet many Leavers will insist that the EU is undemocratic.