1. Negotiating leverage in international affairs
   As long as it's what you want. Trying to please 28 different countries who are on the same side in one negotiation is nightmarish at best
2. Free movement of people
   One of the key points of contention among the British electorate is whether that's a benefit or not.
3. Legal and judicial precedent and statutes Another bone of contention.
4. Common currency. The Euro is still usable in the UK.
   The Euro is hardly legal tender in the UK. It has no more status than the US Dollar i.e if people want it, they'll accept it, and leaving the EU won't change that in the slightest.
5. Cooperation in resolution of political conflicts Mmm, yes, like the EU resolved the Greek elections for them, or how it solved Belgium's year-and-a-half without a government, or fixed Poland's current constitutional crisis.
6. Large influence in the state of European affairs
   Which many would argue is unwelcome meddling
7. More opportunities for people to work/travel 50% youth unemployment in Spain, Greece's fantastic work opportunities, Eastern Europe effectively treated as a source of criminally low-paid labour. On the reverse, UK citizens have very limited work opportunities within the EU due to their appalling lack of foreign language skills.
8. Worker protection
   Regulations either existed before the EU on a comparable level (maternity leave) or were deliberately opted out of (working time directive). etc.