Reasons
The EU has gone against the democratic process numerous times - Ireland, Greece, etc.
It is pretty undemocratic in its very makeup - unelected commissioners with a tier in between which means they cannot be held to account by the electorate
It discriminates against non-EU migrants who may have great skills to offer the country (two-tier system) - I don't support European supremacy - I actually want more migrants but quality over quantity because only quality will contribute to an even better economic contribution
GDP alone is not the single indicator of life quality - a number of countries have lower GDP than the UK yet do better in terms of quality of life
The devastating effect the EU's directives and regulations have had on multiple industries - regulations have crippled SME's who struggle to afford the costs of compliance
Short term pain for long term gain - I believe after an initial shock, which I admit will be the case, the economy will settle down and then eventually thrive
The EU is wasting a lot of money every single year - money that could go to help people in need or could be invested in the poorer nations
Bureaucrats are having a party at our expense whilst again wasting a ridiculous amount of money on vanity projects and events - bloated bureaucracy, jobs for the status quo, etc.
Lack of transparency - the legislation drafting process is incredibly secretive (TTIP, etc.)
One interest rate is not suitable for very diverse economies - the EU is going the wrong way - it wants to centralize power rather than give more control to each nation state
Corporatism - just have a look at who is backing and funding the remain campaign and that will tell you everything you need to know (political elite and economic elite)
Wage compression - in an ideal world, a minimum wage would be enforced, but its not pragmatic to assume that can be enforced as people always find a way to work around the system whether its welfare, tax avoidance or underpaying on salaries - increased competition is leading to abuse by employers, most often at an unskilled level whereby cash in hand is often the way things are done
Jeremy Corbyn had the opportunity to really push for this and instead he gave in to appease the naysayers in the Labour Party to avoid rocking the boat. He has now allowed the entire leave campaign to be hijacked by right wingers with the sole focus on immigration, when that is not the only issue here. The EU goes against the vast majority of what Old Labour stood for, and as someone who would like to return to the days when the likes of Aneurin Bevan and Tony Benn were around, I am disgusted by it.
There are people in the remain campaign who are not voting because of an opinion on the EU, but because of their underlying hatred for people like Nigel Farage and UKIP as a whole. In the long term, these people are going to spite themselves. People are allowing themselves to be driven by emotion, that hatred, rather than looking at the facts and approaching it in a logical way. The reality is, if any true socialist or democrat saw what the EU was actually doing and was, then they would be absolutely disgusted.
I have found that the more research someone on the left does into the EU and how it operates the more they move towards leave. Those making the emotional argument often have no idea who the European commissioners are, no idea how it operates and are no better than those on the right going on about migrants all of the time. The entire debate has quite frankly been embarrassing.