Britain will lose the one thing its economy most depends on - the stock market, as a lot of companies depend on the EU and will probably move their trading to Frankfurt.
We'll lose our rights to work anywhere in the EU, which will hit our children hard in a global economy
We'll lose reciprocal healthcare (and EHICs)
We'll lose all sorts of compensation rights (eg flight delays)
We'll probably lose working hours protections
We might lose maternity leave protection (see US for details) if the govt deem it to cost too much.
We will probably lose a lot of multinational companies who like that they can trade easily with Europe in English
We might gain border autonomy, though most of our post-war immigration was before we joined the EU in the first place.
We will gain the huge EU levy back, but what about the money they invest in us and our companies? I bet that levy doesn't go far in balancing books of UK plc and then where will the money come from? We'll lose a lot of EU workers who do actually pay tax! Leaving the rest of us to find the balance...
We will lose a lot of medical and scientific research (which the UK is currently a world-leader in) as they depend on international collaboration especially with EU countries (where the big pharma are based)
We might crash and burn - in which case we'd have to beg the EU to let us in again and you can imagine what they'd do to us. We'd then lose the £ forever and all rights of veto.
It is worth remembering that the last time a country faced a massive load of political scaremongering (as in "we're being invaded by xyz people/ideology who are taking our jobs and stopping our people from living properly") it was 1930s Germany... they voted to keep out the "communist invaders" and then look what happened. One of the reasons the EU was formed was to make sure it didn't happen again.
Yes, I'm Pro-EU. Ten years ago I would have voted Leave but then I started to think about consequences. With the way our politics is going, we need an external check on the goons honourable persons in power, now the house of Lords is toothless.
And border security? Well, we're an island. We have beaches. If someone undesirable wants to get into the UK and cause havoc, they only need a boat and a dark night. They'll get in if they want to, either way. So that's not a reason to vote Leave.