Will try to list just a few in some semblance of order, assuming literally nothing else bar WTO is in place on Brexit day 1.
The EC has put out a notice saying the EASA certificates pilots and planes require to fly into the EU will no longer be valid if issued by the UK. Every single piece of documentation a plane/pilot requires will need to be re-issued by an EASA Member. That's a lot.
By all accounts, the customs officials at neither Calais nor Dover are remotely equipped, prepared to staffed to handle having to calculate, impose and verify based on risk assessments, tariffs between the EU and UK.
Could be huge delays.
Many EU technical regulations on goods require certification of compliance. The EC put out a notification saying that as of Brexit, certificates from the UK don't count. All covered products will need EU-27 certification. Not fun.
The UK is of course a financial services hub for Europe. Absent a deal, UK based institutions could lose the ability to offer some services to EU citizens. Depending on the host EU Member state, some branches may even lose their licenses.
To operate professionally in the EU as a road transport operator or transport manager, you need a certificate of professional competence from an EU Member authority. As of Brexit, those issued by the UK cease to count.
*** WTO LAW WILL PROVIDE NO IMMEDIATE PROTECTION ***