It's bad.
Importing and Exporting will be more expensive due to tariffs coming back and prices of consumer goods will rise.
Businesses using the UK as a European base are likely to move elsewhere in Europe.
No more social protection through EU law (potentially) or access to European courts.
Political instability (particularly right wing) likely as European influence will not restrict the UK government.
No free movement across Europe and travel more expensive, harder to live or work elsewhere in Europe or split living time between Europe and the UK.
Potentially less or no wartime assistance from Europe, should Britain need military support.
(Just a few points that immediately spring to mind)