Some limitations which I do know of:
* British nationals can’t live and work freely on Europe, so an architect based in London can’t jump on a Eurostar and take a business meeting in Paris by default, they would have to get a visa specific to the country and activity. And their professional qualifications may not be recognised, so would have to operate through local professionals (not sure how that would work in practice).
* Products sold into the EU will have to be recertified for that market, no assumption of regulatory compliance or compatibility. So a lot more red tape and checks in moving products across borders, in particular that will impact food and products built as part of just in time supply chains.
* No ability for British firms to operate in other European countries with equivalent rights as a local company, will have to form subsidiaries instead in many cases.
Would be interested what else people can add.